Something for AR discussions....

[pupdawg]

This was copy n pasted here for a new topic from this older one: http://www.snipershide.com/forum/ubbthreads.php?ubb=showflat&Number=2846609#Post2846609

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: longeye51</div><div class="ubbcode-body">I have a 24" barrell. If I am understanding what your are saying correctly, due to the extra length of my barrell the bolt may start moving back before the bullet leaves the barrell. Is that correct? Or is the correllation the distance from the exiting end of the barrell to the gas tube,e.g., 18" barrell with the gas tube 8" from the exiting end of the barrell caises the bolt to begin to move backwards at the same time as a 24" barrell with the gas tube 8" from the exiting end of the barrell? </div></div>

Yes this MIGHT be true. IF in fact as I said earlier the bullet is still in the bore.

With your 24" barrel there are microseconds of measurable dwell time of the bullet once it passes the gas port until it exits the muzzle.

There is also most likely a minimum amount of dwell time required to let the gas reciprocate into the gas tube and down into the bolt carrier. The gas will not do this reliably if the bullet relieves the chamber pressure (I'll just call it bore pressure) by exiting the muzzle before enough pressure has built up to cycle the bolt group against the buffer system.

So I also assume that this is obviously why a gas port located at rifle length (roughly 12" from the receiver) won't cycle a standard buffer/bolt carrier reliably with a barrel 13" long. (I assume because I have never tried it)

But what I initially was (and still am) curious about is say a 24" barrel (like yours) with a rifle length gas port lets just say 12" leaves another 12" of barrel for bullet "dwell time". Now assuming that the gas port pressure needs to be X PSI to unlock the action, does the gas port see that pressure while the bullet is still in the bore? It has to otherwise the action wouldn't unlock? Chamber pressure drops while the bleed off of pressure into the gas tube is happening otherwise you'd have failure to extract because the case would be pressed up against the chamber still.

So the action must be starting to move while the bullet is still in the bore and is unlocked as soon as the bullet ogive breaks the muzzle crown.

Idk, I'm hoping for someone to sum it up for me because I DONT KNOW for sure... I tried google but couldn't find anything saying something along the lines of "the bullet is in the bore while the action starts to move" or vice versa.

 

[BattleAxe]

Re: Something for AR discussions....

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: pupdawg</div><div class="ubbcode-body">But what I initially was (and still am) curious about is say a 24" barrel (like yours) with a rifle length gas port lets just say 12" leaves another 12" of barrel for bullet "dwell time". Now assuming that the gas port pressure needs to be X PSI to unlock the action, does the gas port see that pressure while the bullet is still in the bore? It has to otherwise the action wouldn't unlock? Chamber pressure drops while the bleed off of pressure into the gas tube is happening otherwise you'd have failure to extract because the case would be pressed up against the chamber still.

So the action must be starting to move while the bullet is still in the bore and is unlocked as soon as the bullet ogive breaks the muzzle crown.

Idk, I'm hoping for someone to sum it up for me because I DONT KNOW for sure... I tried google but couldn't find anything saying something along the lines of "the bullet is in the bore while the action starts to move" or vice versa. </div></div>

When talking 24" barrels we're looking at 6 inches more bullet travel. I would think its <span style="font-style: italic">conceivable</span> that this may increase the odds of the round still being in the bore when the bolt moves but I'm not convinced that this is the case, or that it negatively affects anything. Even if it were, has it moved far enough and/or imposed any force on the weapon capable of disturbing the round? I don't believe so and the reason is recoil.

If you take your upper and grab a locked bolt with your hand you can pull the bolt rearward easily by hand with virtually no effort. Add a spring to the mix and still we're not talking much effort. For this reason I think the gas system has to overcome far more than just spring pressure. The whole rifle assembly is moving violently rearward due to recoil and the mass of the bolt keeps it in its forward position...remember, a body at rest tends to stay at rest.

That said (and for arguement sake) lets say that there is in fact a round still in the bore and the bolt begins to move, and its this reason we have to "drive" a gas gun. My comment about driving the gun being overrated warrants further explanation...

Essentially you're NOT driving anything. A precision auto is generally a 15 lb chunk of aluminum and steel that just experienced a self-contained expolosion while your holding it, and heavy steel parts move violently inside it as a result of this explosion. In a word...the rifle is driving <span style="text-decoration: underline">you</span>. You can't drive it, steer it, or control its movements and the mere mention of "driving" almost implies that this is a proactive event by the shooter. I once bought into this notion but the more I shoot and progress the more I realize that a human absolutely cannot force the rifle do do anything without disturbing where the barrel is pointed...things are simply moving too fast.

So what it "driving"? Driving is a passive event. Every auto moves slightly differently during the firing sequence due to numerous factors. You're simply not strong enough or fast enough to change the outcome of that explosion and direct these movements. What you DO have the ability to control is how you absorb these movements as to not impart even the slighest latteral movement to the weapon during that split second between the hammer being released, and the end of the recoil pulse.

When I have a good day at the range I was simply able to effectively use my body as a shock absorber so that the movements that will <span style="font-style: italic">inevitably</span> occur without my consent, don't cause my trigger pull to move the rifle. The rifle <span style="text-decoration: underline">will</span> move with or without you. "Driving" it is merely the art of ensuring that those movements are exactly the same each time you pull the trigger.

 

[pupdawg]

Re: Something for AR discussions....

Yes I agree with a lot of what you said in the other thread and now in this one. I just figured I'd start a new topic so I don't derail the "buffer accuracy" thread anymore.

So you take the position that the bullet is clear of the muzzle when the action starts to move in an AR. This is what I wanted to know.

Thanks.

 

[Sebben]

Re: Something for AR discussions....

Here i took the slow motion LWRC video and looped it a few times as well as slowed it down so you can see what actually goes on in that fraction of a sec.

the first slow down is at .5x then 1x a few times then another .5x and the last one is .25x speed.

<object width="425" height="350"> <param name="movie" value="http://www.youtube.com/v/wECpY9MVZI4"></param> <param name="wmode" value="transparent"></param> <embed src="http://www.youtube.com/v/wECpY9MVZI4" type="application/x-shockwave-flash" wmode="transparent" width="425" height="350"> </embed></object>

Here is the full stroke of the action.
<object width="425" height="350"> <param name="movie" value="http://www.youtube.com/v/WuUwFdBMaNw&feature=related"></param> <param name="wmode" value="transparent"></param> <embed src="http://www.youtube.com/v/WuUwFdBMaNw&feature=related" type="application/x-shockwave-flash" wmode="transparent" width="425" height="350"> </embed></object>

In the first video you can see the piston does not move until the bullet has left not just the barrel but the flash hider. I know you are talking about DI guns but the effect of the action cycling before the bullet leaves the barrel (if this were the case) would be even more noticeable in a piston gun due the the fact that the gas does not have to travel down the gas tube and apply pressure to the bolt carrier group. It only has to move a less than an inch to apply pressure to the piston which is resting on the bolt carrier.

My question is since the bullet leaves the bbl before the action starts to move, and pressure drop off is suppose to be immediate once the bullet leaves the barrel. Then how the hell is the gun suppose to work in the first place?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: The Mechanic</div><div class="ubbcode-body">If the gas jet was the only or at least most effect on recoil, you would never get a 1911 to unlock upon firing.

</div></div>

See that .2ms time between the bullet exit and the powder gas exit? That's how they work. That .2ms of pressure still in the barrel with no bullet allows the action to be cycled with no bullet in the bore. There is still even pressure in the bore as the bolt is cycling. If you have ever fired in a dark place you will sometimes see sparks/embers coming out of the chamber and ejection port. So that means pressure is still dropping as the round is being extracted and has only completely dropped once the spent cartridge is removed. So in short. If your timing is correct and your gas ports are the right size your bullet is out of the muzzle by the time the action moves.

 

[Afghanvet09]

Re: Something for AR discussions....

"In the first video you can see the piston does not move until the bullet has left not just the barrel but the flash hider."

Don't know how to do "select quotes" on SH but regarding the above observation- I don't know..when the piston starts to move you see a vertical dark line form. I am thinking I see that form just before bullet exit. Can't simultaneous view muzzle and piston, using peripheral vision and could be mistaken.

 

[Sebben]

Re: Something for AR discussions....

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Guard Marksman</div><div class="ubbcode-body">"In the first video you can see the piston does not move until the bullet has left not just the barrel but the flash hider."

Don't know how to do "select quotes" on SH but regarding the above observation- I don't know..when the piston starts to move you see a vertical dark line form. I am thinking I see that form just before bullet exit. Can't simultaneous view muzzle and piston, using peripheral vision and could be mistaken. </div></div>

What you see is a small amount of gas exiting the separation between the piston cup and gas block before the movement.

 

[Sebben]

Re: Something for AR discussions....

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: pupdawg</div><div class="ubbcode-body">So the action must be starting to move while the bullet is still in the bore and is unlocked as soon as the bullet ogive breaks the muzzle crown.</div></div>

Doesn't GAP only offer 22 inch bbl with mid length gas? Maybe GAP would have some input as to why this is. If I were to speculate 22" is close to the limit for the timing to work properly with a mid length gas system. If the barrel were any longer then you would have to go to a rifle length gas system for the rifle to work properly and have it be accurate.

Here, this video explains more in depth the operation of ARs and timing.
<object width="425" height="350"> <param name="movie" value="http://www.youtube.com/v/AA0rkdpk1Hw"></param> <param name="wmode" value="transparent"></param> <embed src="http://www.youtube.com/v/AA0rkdpk1Hw" type="application/x-shockwave-flash" wmode="transparent" width="425" height="350"> </embed></object>

 

[Rovieairto]

Re: Something for AR discussions....

Yes, pressure begins building in the piston chamber as soon as the bullet has passed the gas port.
As soon as the bullet exits, pressure in the bore, and at the gas port, begin to drop rapidly.
The gas port, on your example, would see around 0.6 milliseconds of pressure, however the gas system contains this pressure for about 2.4 millisecond because of the delay in pressure rise (inherent to the gas system).
The majority of the carrier acceleration occurs over a period of about 1 millisecond, but the whole event lasts 2-2.5 times as long.
The carrier begins this acceleration while the bullet is still in the barrel, but continues to accelerate, even after the pressure drops after bullet exit.
The gas system pressure only sees a very tiny drop when the gas port is exposed (for a 223 you'd be looking at losing around 20 ft x lbs of bullet energy due to this siphoning).
Where the AR really cuts it close is when the bolt begins to rotate, chamber pressure needs to be low.
The only saving grace is that the AR gas system alleviates some rearward bolt thrust during bolt rotation as pressure in the piston chamber applies forward force to the bolt, and rearward force to the carrier.
The initial recoil velocity of the rifle must also be considered as the recip mass gets a running start just prior to peak acceleration.
For the large frame AR's this running start is significant.

 

[Rovieairto]

Re: Something for AR discussions....

SENSITIVITY STUDY OF RIFLE GAS SYSTEMS:
http://www.dtic.mil/cgi-bin/GetTRDoc?Location=U2&doc=GetTRDoc.pdf&AD=AD0880431

The math and empirical data behind the M-16 and AR-18 gas systems.
They are both piston operated guns with very similar gas dynamics.
The piston chamber pressure has a far bigger delay in the M-16, but otherwise they are almost identical systems.

A STUDY OF PRESSURE TIME CURVES FOR SMALL CALIBER AMMUNITION:
http://dodreports.com/pdf/ada364826.pdf

More good data.
I have some excellent slow motion video that is worth more than these two studies.
The bullet is measurable in distance and therefor a handy time-line can be established for events to take place relative to the bullet's exit, Like how long pressure takes to reverse flow through the gas port etc....

 

[Sebben]

Re: Something for AR discussions....

@Hamilton. So even if the bolt carrier moves a slight amount with the bullet still in the bore. Wouldn't the effect on accuracy be almost immeasurable, due to the fact that the BCG moves in line with the bore imparting only linear force for that split second?

 

[Rovieairto]

Re: Something for AR discussions....

That's what I'd intuitively believe.
If though experimentation, (which I haven't conducted), reveals that there is a relationship between the buffer and accuracy, I'd bet that it likely has an indirect one.
I do know that changing the mass characteristics of a rifle will effect it's natural POI.
The rifle is accelerating rearward while the bullet is accelerating forward.
The bullet will be effected by how it's launching platform moves as it's in transit.
Adding a heavy scope is a prime example of how to change the POI with an off-set mass.

I just can't see how the buffer will effect much of anything during this first millisecond.
Though I've seen much weirder non-intuitive relationships before.

Still, I will never argue what is.
If their is a relationship, well,...I'll be damned.

 

[Sebben]

Re: Something for AR discussions....

I think the line is being blurred between bolt gun accuracy and precision AR accuracy. Aside from world record setting. Both are capable of 1/2 to 1/4 MOA. Which most, by most I mean 95% of shooters can't consistently achieve. That being said, any negligible accuracy loss caused by moving parts, gas systems and or any other voodoo is just that, negligible voodoo! If people are trying to squeeze every bit of accuracy out of a $800 AR that's a 1.5 - 2 MOA plinker than I hate to break it to them but they bought the wrong gun.

 

[pupdawg]

Re: Something for AR discussions....

First I want to say thanks to all that replied.

Lotsa good info and a good discussion.

Whether or not the bullet is still in the bore when the carrier starts to unlock IS irrelevant to accurately shooting an AR. I know this to be true as do all of you because we have or have seen AR platforms capable of sub MOA accuracy. Its nothing uncommon.

This is merely just a curiosity thing for me. I personally have no problems shooting my AR into sub MOA/ MOA. I personally dont measure group sizes on paper because it means nothing to me, unless someone asks so I need some reference.

The AR design with its reciprocating mass in line with the bore helps keep this movement more linear as Sebben has said earlier and while this movement is negligible its also part of the reason why precision shooting an AR is a little different than a bolt gun. Fundamentaly being the same, but dynamically different.

Great discussion guys.

Again I appreciate all the info and videos you guys gave in this thread. Very interesting.

 

[Afghanvet09]

Re: Something for AR discussions....

Agreed interesting discussion. Not a engineer but my own personal experience (like many) is there obviously a difference between shooting a bolt and gas/piston and most would say a bolt is easier and more consistant.

I then look to the the difference between to the 2 platforms as to explain why. One platform (BOLT) is a locked bolt with no gas or piston system and no buffer/buffer spring VS. another platform (AR) with a moving bolt with gas or piston and buffer. Granted a bolt gun built for accuracy typically has a varmit or heavier barrel and this aids accuracy..

intuitively i've believed that the system in the latter (the AR), a more complex system, with it's actions and moving parts, in some manner contributes to the greater difficulty by creating disturbance(s) that exert some negative effect that does not occur with a bolt platform.

i don't know calculus, and please correct me if I'm wrong here but doesn't trig tells us 1/100th of an inch movement at the muzzle translates to 1" of movement 100 yards away? Doesn't take much to blow a group if that's correct..

 

[longeye51]

Re: Something for AR discussions....

Based on your explanations and the video, I do not understand how shooting using the Tubb spring is more accurate than using the stock spring. I know from testing it that it does. I also know it changes the zero on my gun by 2 inches at 100 yards. That means it cannot be a placebo effect.

In regard to how the bolt returns after the bullet leaves the barrell, I think there is a simple explanation. The force of the explosion in the barrell is so great that the hole at the end of the barrell is not sufficient to release the full power of the explosion. As a result, gas is forced through the tube to drive the bolt back as well. I am too ignorant mathematically to prove that, but maybe one of you guys can. I know if you set a bomb off in a concret structure,the structure is stronger than the force of the explosion and the structure has openings; the force of the bomb will come out of every available opening, not just the first one to which it comes.

 

[BattleAxe]

Re: Something for AR discussions....

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: longeye51</div><div class="ubbcode-body"> I do not understand how shooting using the Tubb spring is more accurate than using the stock spring. I know from testing it that it does. I also know it changes the zero on my gun by 2 inches at 100 yards. That means it cannot be a placebo effect. </div></div>

I'm not saying this to offend you so please take none...

I've seen your test and you're making a lot of <span style="font-style: italic">assumptions</span> based on an outcome that could have been purely chance. Looking at those targets you can assign 20 different versions of what <span style="text-decoration: underline">appeared</span> to have happened, none of which is necessarily what <span style="text-decoration: underline">actually </span>happened.

I shot at least one sub-minute group today in constantly changing winds at 600 and again at 850 yards...that would be like me saying that my factory spring is superior based on that performance.

You're essentially painting the target around the bullet hole when you make those kind of assumptions. If you were the "Tiger Woods" of precision shooting I may give more weight to such claims but you admitted yourself that you haven't shot since May and we all know precision shooting is a perishable skill and therefore your results may simply be chance.

All that said I believe that you shot the targets as you've said but I don't believe for a second that they mean anything more than you shot a few targets and achieved some mildly interesting results that warrant wayyyyyyyy more investigation to have even a remote change of confirming them.

 

[Sebben]

Re: Something for AR discussions....

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: longeye51</div><div class="ubbcode-body">Based on your explanations and the video, I do not understand how shooting using the Tubb spring is more accurate than using the stock spring. I know from testing it that it does. I also know it changes the zero on my gun by 2 inches at 100 yards. That means it cannot be a placebo effect.

In regard to how the bolt returns after the bullet leaves the barrell, I think there is a simple explanation. The force of the explosion in the barrell is so great that the hole at the end of the barrell is not sufficient to release the full power of the explosion. As a result, gas is forced through the tube to drive the bolt back as well. I am too ignorant mathematically to prove that, but maybe one of you guys can. I know if you set a bomb off in a concret structure,the structure is stronger than the force of the explosion and the structure has openings; the force of the bomb will come out of every available opening, not just the first one to which it comes. </div></div>

I find the test you did to be inconclusive. There are way to many variables. The human being one of them. Not just you, anyone. To do this test right it would have to be done with no wind and no human and no factory loads. The more variables you remove the more accurate the test would be. So it would be an indoor test in a led sled with the trigger pulled remotely.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: rsilvers</div><div class="ubbcode-body">
It would take a lot of rounds to prove a difference. </div></div>

To do this id say maybe 1000 rounds per spring. 100 10 shot groups. Simply because you also want to test spring deterioration over round count.

Could the different resonance between springs possibly have an effect on accuracy / POI? I had this idea last night and you can feel the entire gun vibrate when you pull the trigger dry. You can even feel it at the end of the barrel.

 

[longeye51]

Re: Something for AR discussions....

No offense taken. What I did not tell you was that I always check my zero at 100 yards prior to and subsequent to the long range shooting that takes place in between. I never miss at 100 yards, <span style="font-weight: bold"><span style="text-decoration: underline">never</span></span>. I can shoot the buttons off your shirt at 100 yards under any conditions (short of you running so that I cannot see them; Under those conditions, I could still hit you, just not the buttons).

I made sure the gun was zeroed at 100 yard with the Armalite spring before I began conducting the test. As usual, I put three shots in the bulls eye, sub moa. I checked it after the test with the Tubb Spring.

The first three shots were again sum moa, but were 2 inches above the bullseye. I could not beleive it and carefully placed a couple of more shots and got the same result. I then purposely aimed at the yellow ring below the red bullseye for the next 3 shots. I placed 3 shots at 1/2 moa in the center of the bullseye.

What I am telling you is that something other than dumb shooting luck (having a better day than normal) is going on. I am not a beginner nor do I have subquality equipment. I know the instant I pull the trigger whether the shot is going to be decent or not. You develop a feel for it after doing it for years.

Galileo was persecuted by all the learned scientist of his time for asserting the world revolved around the sun (heliocentrism), not vice versa. We now accept that truth without pause. Sometimes science and math cannot explain things we know to be true. That is simply because of our limited understanding of the laws of science and/or math at the time. If you notice, people often refer to the working of things they do not understand as an art. Once they figure out the laws that make it work and how it functions within them, it becomes a science. Obviously, I nor anyone else are going to change your mind (and that's okay), but it does not bother me. I know.

 

[BattleAxe]

Re: Something for AR discussions....

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: longeye51</div><div class="ubbcode-body">I made sure the gun was zeroed at 100 yard with the Armalite spring before I began conducting the test. As usual, I put three shots in the bulls eye, sub moa. I checked it after the test with the Tubb Spring.</div></div>

Then surely you can understand our skepticism and this statement doesn't help.... 3 shots in the bull with the factory spring at 100 yards, you can't pull of the same feat at 600 and its the springs fault? How is that?

 

[SD26]

Re: Something for AR discussions....

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: BattleAxe</div><div class="ubbcode-body">
Then surely you can understand our skepticism and this statement doesn't help.... 3 shots in the bull with the factory spring at 100 yards, you can't pull of the same feat at 600 and its the springs fault? How is that? </div></div>
I don't think he answered the "how is that" question, but he has provided data. Is it the "spring's fault"? Seems to only lead to needing more data.

 

[RoosterShooter]

Re: Something for AR discussions....

In being less analytical, and not "Over analyzing" the issue I will explain it this way.

A standard .224 diameter bullet will travel at around 3 times the speed of sound. A standard BCG will travel at around 1 time the speed of sound.

So, to clarify the issue the bullet has already left the barrel before the BCG makes one complete cycle.

Over analyzing the gas impingement system only creates more non-existent problems.

Cycling issues with 16" or shorter barrels can be fixed by slowing down the BCG. IE: heavier buffer spring, heavier buffer, adjustable gas block, or all three!


From my experience piston systems on an AR-15 platform cause more issues than they resolve over time. This is why the US Military has failed to switch from the DGI system even after several field tests.

 

[-Nick-]

Re: Something for AR discussions....

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Sebben</div><div class="ubbcode-body">
Doesn't GAP only offer 22 inch bbl with mid length gas?
</div></div>

I have the GAP-10 in .308 the 22" barrel on the mid-length system and it hits spot on with no operational or accuracy issues.

 

[longeye51]

Re: Something for AR discussions....

I never said it was the spring's fault. I just said the pattern is tighter with one than the other. I did provide the data, albeight in multiple posts (the best I did with the Armalite was 5/8 moa at 100 yards, I did 1/2 moa at 100 yards with the Tubb). That equates to tighter patterns at longer distances.

Battleaxe, you know that at 600 yds, anything within a 6 inch circle is still sub moa. At 900 yds, it would be anything within a 9 inch circle, etc... I have done that with the Armalite spring in the AR. I simply did a test to see if a gun that was already capable of a sub moa pattern could get any tighter of a pattern with a Tubb spring. It did.

You may kill the messenger, but that won't change the message. For some reason, unknown to me, the Tubb spring holds a tighter pattern. Some additional data to add to the test data, the ammunition was all from the same run. I never considered anything but an Armalite spring before reading this post. I also do not nor have worked for Tubb or know anyone who does. I do not have nor have ever born a grudge against Armalite, anyone who works there or any of its products. In fact, I have always considered them to be top of the line in gas guns.

 

[Rovieairto]

Re: Something for AR discussions....

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Broker</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Sebben</div><div class="ubbcode-body">
Doesn't GAP only offer 22 inch bbl with mid length gas?
</div></div>

I have the GAP-10 in .308 the 22" barrel on the mid-length system and it hits spot on with no operational or accuracy issues. </div></div>

It's good to see logic in the face of ignorance.
That gas position is probably ideal for that caliber/powder volume/burn rate, almost regardless of the barrel length.
You should get more consistent cycle energy with a wider selection of bullet velocities, and burn rates.
My 260 rem AR has a 20" barrel with rifle length gas.
All my H4831 loads cycle the rifle with enough energy.
All my H4350 (faster propellant) loads give insufficient cycle energy, even at the top pressure wise although it almost gets there.
I assume it's because the port pressure is higher for the slower propellant, giving higher cycle energy.
The slower powder loads also display lower peak pressure, even at the top.
If it were moved closer to the breach, with a smaller port, it should get similar cycle energy with either loads, that's what I plan on doing for my next barrel anyways.
I'm sure very little testing was conducted with this set-up, but it is/was a factory gun.

 

[BattleAxe]

Re: Something for AR discussions....

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: longeye51</div><div class="ubbcode-body">You may kill the messenger, but that won't change the message. For some reason, unknown to me, the Tubb spring holds a tighter pattern. </div></div>

I'm not trying to kill the messenger. Out of all my rifles I shoot my AR10 the most, and if there was something that I could do to it for $30 to make the weapon more accurate I'd be the 1st one in line.

Problem for me is that you haven't proven anything, yet you're making definative performance statements based on very little data. In fact it almost appears as if you're trying to pass off what is clearly speculation as fact. That's not meant as an insult but you are in fact speculating as to the reason for the tighter group.

One minute you're "rusty", the next minute you never miss and can shoot the buttons off a shirt", then that description changes to "the best I could do without the Tubbs is 5/8 moa. The significance here is that you need to remember where you are. I know exactly what the best autos can do. I also know that if I had an auto of the calibur that you describe (your auto before the spring), I'd know with certainty that it wasn't broken.

You make it too hard to take you seriously. That's hardly my fault dude. I'd have taken you more seriously had you said that the results were interesting and promising but you needed to see if the resuls were consistent and repeatable over time. You didn't do that. You proclaimed to the world that it absolutely makes a significant difference based on a few targets. Again...its just not believable the way its presented.

 

[longeye51]

Re: Something for AR discussions....

Battleaxe,

Since I am so hard to take seriously, buy a $30 spring, put it in your favorite gun and test it yourself anyway you see fit. Report back to us with your data and results. Don't just sit on the sidelines and criticize. That is too easy.

I personally will continue experiment, try new things and hopefully improve my skills while doing so. If I don't, at least I can say I gave it a shot. I did not spend the time and effort to do the test I did to argue with you, I did it in hopes of shedding some light in regard to a legitimate question.

I put the results of the test out and as much data as I could into how the results came about. The opinions on the results are my own. I may have arrived at my opinions prematurely and/or innacurately, but they are mine and they were arrived at by actually doing something.

I apologize to everyone who has had to read all of this exchange from deviating from the path to gain knowledge that I started out on. I am now returning to it.

 

[longeye51]

Re: Something for AR discussions....

Pupdawg,

Thanks for moving the string over. I have learned a lot from everyone's input in this discussion. The videos and links were very helpful. Thanks to everyone who took the time to contribute to what has been a helpful learning experience for me.

 

[Afghanvet09]

Re: Something for AR discussions....

Gentlemen, there's no practical way to finance a truly scientific study of this (tubbs spring) for the group.

I don't mind the personal opinions, anectdotal use, individual field testing and results from any skilled, intelligent individual who is not a pitch man or shill for these products. Most of us are schooled enough to can weigh the results under the circumstances presented.

We do this everyday on SH on a variety of topics and I wouldn't discourage it because in the absence of anything, I find the info somewhat helpful.

 

[PLeighton]

Re: Something for AR discussions....

Short answer:

Get one of these:

LMT Enhanced Full Auto Bolt Carrier Group
The LMT Enhanced BC retards the timing of the unlocking sequence by altering the cam angle and adding two additional high pressure vent ports.

IMO:
The added reliability that they bring also completely negates the GP vs. DI argument.
(Minus ease of cleaning anyway...)

I've been running the LMT Enhanced BCG in two rifles for several years now.
They are expensive but well worth the coin.
Do your own research though.
Feel free to hit me up if you have any questions.

Here is an excellent explanation of the LMT eBCG:
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Bill Alexander</div><div class="ubbcode-body">
Let us perhaps be a little more precise with regard to the details of the LMT enhanced bolt and carrier system. As shown the details are very often misinterpreted.

In the first instance one must understand why the system exists, more particularly in the context that the design is brilliant in its execution, but as reported frequently on the errornet is not always reliable. The design basis for the assembly is specifically to enhance the durability of the M4 configuration as currently issued. This gun is to some extent unbalanced. Port pressures running military grade ammunition are well in excess of the design levels which leads to very high carrier acceleration rates (+22 f/s/s). this combined with chamber pressure drop often creates the undesirable condition that the bolt commences to unlock while still subject to a degree of head thrust from the cartridge case. The result is that the lugs are subject to a biaxial loading of both shear and bending.

The LMT system being both the carrier and the bolt seeks to operate directly in this gun and elleviate symptoms. The carrier is set up with a longer delay during the initial portion of its movement. To facilitate this longer cam path and movement of the bolt forwards in the carrier, the front edge of the carrier is extended such that it still continues to retain the extractor pivot pin. Additional exhaust vents act to drop the piston pressure faster and to relieve any blowby at the tail of the bolt.

The bolt itself is of particular interest. The function of the dual spring extractor is frequently misinterpreted as an attempt to add spring force to the extractor claw. Rather it reduces the fatigue that the extractor spring(s) undergo by allowing the use of longer springs with lower K values; the % relative compression during the movement of the extractor is reduced. Remember that additional extractor force is not required now that the carrier is slowing the extraction cycle. The mitigation of stress in the bolt is accomplished in several ways. Material is the least visible change but is important to the design. The traditional Carpenter 158 is abandoned, being replaced by a significantly tougher grade from a different manufacturer. The lugs themselves are generously radiused between lugs and at the rear the diameter is actually reduced to allow a larger transition radius to be machined. The incorrectly identified sand cuts on the lugs are stress relief cuts. These allow any individual lug to elastically deform and give a smoother load over the contact patch. While this type of feature is very difficult to calculate and even more difficult to implement it helps to place the lug in a true shear load rather than amplify the bending moment. As noted the lug opposite the extractor is relieved. This feature prevents the unequal transfer of load to the two opposite lugs but I would argue that the stress relief groove already in place largely accomplishes this purpose. This is a academic quibble so I will bow to LMT in this respect. There is one additional feature that can be found in the bolt, but I am not at liberty to disclose the detail.

When considering whether to use the LMT parts one must consider the weapon. Correctly ported guns will derive little or no benefit from a carrier that is specifically set up to absorb excessive port pressures and some degree of residual case pressure. If not sufficiently gassed the reliability will suffer. This is not a fault of the carrier rather a mistake made in the application so be careful with simple substitutions. The bolt itself is exemplary. If not constrained by a $ value the bolt is a worthy addition to any rifle and will do nothing but enhance the durability of this part of the system.

Bill Alexander </div></div>
Here are a couple threads that discuss the LMT eBCG:
http://creativepromotionalmarketing.com/showthread.php?s=5636716c3873327494cbb88f8c1aa28f&t=3379
http://mail.m4carbine.net/showthread.php?t=3379

 

[pupdawg]

Re: Something for AR discussions....

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: RoosterShooter</div><div class="ubbcode-body">In being less analytical, and not "Over analyzing" the issue I will explain it this way.

A standard .224 diameter bullet will travel at around 3 times the speed of sound. A standard BCG will travel at around 1 time the speed of sound.

So, to clarify the issue the bullet has already left the barrel before the BCG makes one complete cycle.

Over analyzing the gas impingement system only creates more non-existent problems.

Cycling issues with 16" or shorter barrels can be fixed by slowing down the BCG. IE: heavier buffer spring, heavier buffer, adjustable gas block, or all three!


From my experience piston systems on an AR-15 platform cause more issues than they resolve over time. This is why the US Military has failed to switch from the DGI system even after several field tests.
</div></div>

I don't think I was over analyzing anything when I asked "Is the bullet still in the bore when the action STARTS to move?" I asked because I didn't know the answer and was curious about it.

Now the question I asked has nothing to do with the carrier group doing a complete cycle. Everyone knows that the bullet is at least 200 yards by then.

The question also has nothing to do with cycling issues on 16" or shorter barrels. Cycling issues are directly related to gas port size and or location.

The question also has nothing to do with shooting an AR accurately nor piston vs DI in an AR15.

It was just a question that of which was answered with video's and loose info all pointing in the direction of "the bullet is clear of the bore when the action starts to move".

 

[RingSteel]

Re: Something for AR discussions....

Lots of great input on this thread. Subscribed. Carry on.

 

[Afghanvet09]

Re: Something for AR discussions....

Regarding my earlier thread post and suggestion of greater complexity forces of a gas v. bolt gun that contributes to making former more difficult to master than the latter :

An AR bolt may not move before bullet departure, and an AR bolt/carrier may be in line force with movement as pointed out, but a high piston gas system and gas tube system (i.e., like with AK pistons, AR pistons or AR gas tubes guns) do themselves represent "out-of-line" vector forces. They do exert their effects before bullet departure. They are "out of line forces" and not present on a bolt gun, which has no such system.

Such out of line forces can have adverse effects on accuracy.

 

[Rovieairto]

Re: Something for AR discussions....

It would be easy to test this with a gas system that can be turned off.
The AR will experience a peak of around 90 lbs forward thrust at the gas block, at a moment arm that could be easily measured.
This force x moment will cause a torque that will manifest as bending stresses transmitted in either direction.
These stresses will easily outrun the bullet, so some flexure should to.
I'm going to test this as soon as I acquire a descent gas block with a real cut-off.
I don't expect to be blown away by the results, but I can't seem to squeeze a real 1/2 moa out of my large frame AR's.
I've shot many sub 1/2 moa 5 shot groups with them but they are not 1/2 moa guns for sure.
With an AR15 built for accuracy, it's very realistic for me to get a real 1/2 moa shooter (or better).
So:
I was going to build a palma gun out of my side charger 308 AR.
Put a quality 26"-30" barrel on with no gas port drilled, and I'd have a straight pull AR.
When I'm done with it I could turn it back into an autoloader.
The trigger lock time will still suck (compared to a bolt-gun), but I figured I could have a nice set-up for just the cost of a custom barrel.
It should only be more accurate with no gas.
I need a least 1/2 moa out of it.
Anyone ever hear of someone doing something similar to this?

 

[pupdawg]

Re: Something for AR discussions....

Sounds interesting.

I'm sure it will work pretty well.

Figure a 30" tube with maybe a 1-12 twist for the 155 palma smk?

 

[longeye51]

Re: Something for AR discussions....

I do not know anyone that has tried that, but it does sound interesting. Let us know the results. Even the dyed in the wool bolt gun guys ought to appreciate the increased capacity of a 25 round mag (not putting down bolt guns; I consider guns to be like tools; each has its own purpose; so I have some bolt and gas). What are the specs on the barrell you currently have and the best moa you have gotten with it? Just curious if it is close enough to what you are going to put on it to make a realistic comparison of the change in moa.

 

[Rovieairto]

Re: Something for AR discussions....

It's an 18.5" SS, 1:10, criterion barrel.
I consider it to be a true 1 MOA rifle out to 800 yards. (usually less)
At 1000 yards though, I'm not surprised to see 1 to 1.5 moa 5 to 10 shot groups.
Most Palma gun barrels are around 30" and shoot tight.
I was thinking more around 26-27" Lilja 1:12 or higher, medium palma contour.

I need more velocity, on a stiffer barrel.
I think 1/2 moa should be realistically attainable.

 

[AK Sgt Maj]

Re: Something for AR discussions....

I am a High Power shooter, and I shoot Bolt guns at long range (800, 900 and 1000 yards), and AR-15 platforms out to 600 yards. One of the things that we try to do, is slowdown the bolt unlocking by use of different springs, heavier bolts, adding weight to the bolt or changing the gas port, either the size or location. I have a Master classification in High Power, Mid-Range and Long Range, so I have some idea of what I am talking about. Out to 600, no bolt gun can beat an AR, it is the shooter who will get the higher score. I also shoot various A2's in 3 gun where you want the action to cycle fast, because they don't require the same accuracy, speed is what counts. On my 26" Spacegun, the gas port is moved out 2", and the cycleing is noticeby slower and the accuracy is higher. At the long range, I have not seen a gas gun keep up with the bolt guns, because the AR-15 platform can not handle a cartridge that will keep up with the bolt guns and the AR-10 platforms cannot keep up with either, in terms of accuracy, even thought Tubbs won the Nationals with one, one year, he gave up on it because of the problems with it, but the AR-15 platform has won the Nationals different years. The whole thing is what are you looking for in your rifle, not what someone else thinks you should be using.

 

[longeye51]

Re: Something for AR discussions....

AK Sgt Maj,

I would like to pick your brain regarding some of your knowledge/experience. I have an AR-10 that I like because it gives me speed and distance. I just recently put a Tubb spring in it that appears to have helped tighten the pattern, but am always looking for new ways to continue to tighten the pattern. I am shooting a 24" stainless bull barrell. What type/brand of heavier bolt would you recommend for use in conjunction with the Tubb spring?

 

[AK Sgt Maj]

Re: Something for AR discussions....

I don't know if the AR-10's bolts are different in weight. Tubb has a carrier weight system for the AR-15, but I don't know if it will fit in an AR-10 bolt. It is a carrier that fits in the rear of the bolt, and it comes with 2 weights, one is SS and the other is tunston carbide. I used one for quite awhile, but when I got my Spacegun with the gas port moved forward 2", I liked that better. When Tubb was messing with the SR-25, I know that he moved the gas port forward. A gunsmith could plug your gas port and move it forward, then the gas block would need to be moved and a longer gas tube installed. White Oak Precision could probably do that for you and they also have a 14" handguard for both the AR-15 and the AR-10. That's who built my upper for me and I built the lower. I use a M1A for my .308 gas gun.

 

[longeye51]

Re: Something for AR discussions....

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: AK Sgt Maj</div><div class="ubbcode-body">I don't know if the AR-10's bolts are different in weight. Tubb has a carrier weight system for the AR-15, but I don't know if it will fit in an AR-10 bolt. It is a carrier that fits in the rear of the bolt, and it comes with 2 weights, one is SS and the other is tunston carbide. I used one for quite awhile, but when I got my Spacegun with the gas port moved forward 2", I liked that better. When Tubb was messing with the SR-25, I know that he moved the gas port forward. A gunsmith could plug your gas port and move it forward, then the gas block would need to be moved and a longer gas tube installed. White Oak Precision could probably do that for you and they also have a 14" handguard for both the AR-15 and the AR-10. That's who built my upper for me and I built the lower. I use a M1A for my .308 gas gun. </div></div>

Thanks, I appreciate the tips