What PSI to get an AR-15 action to cycle?

[Fx51LP308]

I completed my 1st build of an AR-15 this year, and I have just begun to test fire it. It does fire. It does go BANG! But I can't get it to cycle automatically. I keep having to pull the charging handle to get it to cycle. It cycles perfectly when I operate the charging handle manually... spent brass is ejected and the next cartridge is loaded into battery. And again, it goes BANG! It just won't cycle automatically.

Most likely, a gas block/tube/key/port alignment issue. So, I've already disassembled it once in an attempt to re-align things. I have yet to try my 2nd round of test firing. The difficulty I have is that I have no means to test fire except on the range (in my case, JTAC, 1hr away). I have no place where I can test fire locally. My county does not allow discharge of firearms (except for SD) in an area where residential units are closer together than one mile. So I thought, if there was a way to use "compressed air" down the barrel to simulate the gas blow of a shot, I could rig up something that I could place over the barrel's muzzle end (I'd take off the flash suppressor of course) and see if I can get the action to cycle with that.

But what PSI would I need for that? What PSI, for example, is generated by an XM855 Green Tip (62gr) when fired, such that it causes the action to cycle? I'm only trying to simulate fire so I don't have to keep going back and forth to my range to test fire for real. But, if I have to, I have to.

Appreciate any/all reasonable advice.


Thanks!

 

[Mbaysinger89]

Only about 55k PSI. Your local Home Depot probably has a compressor that can achieve that without exploding.

 

[drew_eckhardt]

You need about 15,000 psi with a rifle gas system and 30,000 with a carbine setup.

Neither is achievable even with a SCUBA compressor.

If you could figure out a workaround, you'd also need a valve which can limit the pressure burst to the fraction of a millisecond a bullet spends between the gas port and muzzle.

I'd forget trying to simulate firing, then use this as an excuse to buy a dimpling jig to guarantee alignment and pin gauge set to measure my gas port to see if it's in-spec.

From http://www.ar15barrels.com/prod/operation.shtml

 

[UpSideDown]

You need to shoot it. Pressure is only part of the equation, there's no viable test cell that doesn't involve firing a round.

Is it cycling at all? Is the bolt coming back partway? If not you probably have an improperly dimpled barrel where the gas block isn't lining up with the port, which is easy to check with some disassembly to at least eyeball if that's off. Could also have an improperly installed gas tube that's dumping all the gas, if it's not seated fully before the pin is installed.

 

[GreenGO Juan]

Are you right on top of your neighbors?

Lawn mower in the garage with the carburetor exposed and the muffler removed. 5 gallon bucket full of sand with a steel plate at the bottom, surrounded by sand bags in a bathtub with an exhaust fan running, it doesn't take much sand to stop things. Start the lawn mower. Lawn mowers can back fire and make loud popping sounds. If you have a lawn mower that runs like shit, all the better. Hopefully you get that lawn mower running right.

 

[Fx51LP308]

You need about 15,000 psi with a rifle gas system and 30,000 with a carbine setup.

Neither is achievable even with a SCUBA compressor.

If you could figure out a workaround, you'd also need a valve which can limit the pressure burst to the fraction of a millisecond a bullet spends between the gas port and muzzle.

I'd forget trying to simulate firing, then use this as an excuse to buy a dimpling jig to guarantee alignment and pin gauge set to measure my gas port to see if it's in-spec.

From http://www.ar15barrels.com/prod/operation.shtml

View attachment 7691861

I'll be buying the dimpling Jig anyway, but if it's an issue with the key in the upper and the BCG, then I have other problems. I don't think so. I think it's a gas block/port alignment issue.

 

[Fx51LP308]

Are you right on top of your neighbors?

Yes, sadly. Definitely not some place where I can do any kind of practice or diagnostic firing.

Lawn mower in the garage with the carburetor exposed and the muffler removed. 5 gallon bucket full of sand with a steel plate at the bottom, surrounded by sand bags in a bathtub with an exhaust fan running, it doesn't take much sand to stop things. Start the lawn mower. Lawn mowers can back fire and make loud popping sounds. If you have a lawn mower that runs like shit, all the better. Hopefully you get that lawn mower running right.

Sadly, it's one of those lawn mowers with a "safety switch" that will kill the motor if you don't keep holding it down. I suppose I could try to "duct tape" it, but we'll see. And I'd get in trouble with the HOH for excessive Noise, anyway.

 

[UpSideDown]

National Forest, BLM land, or a farmer's land with their permission.

 

[Fx51LP308]

You need to shoot it. Pressure is only part of the equation, there's no viable test cell that doesn't involve firing a round.

Is it cycling at all? Is the bolt coming back partway? If not you probably have an improperly dimpled barrel where the gas block isn't lining up with the port, which is easy to check with some disassembly to at least eyeball if that's off. Could also have an improperly installed gas tube that's dumping all the gas, if it's not seated fully before the pin is installed.

No, the BCG won't budge at all. It's doing nothing. But, again, if I pull the charging handle manually, it does everything it should.

 

[Fx51LP308]

National Forest, BLM land, or a farmer's land with their permission.

In that case, I may as well go to my range. It's a lot closer. I just have to rent a car each time I go.

 

[whatsupdoc]

Watch this video so instead of BANG you will get BANG BANG

 

[GreenGO Juan]

Most likely, a gas block/tube/key/port alignment issue. So, I've already disassembled it once in an attempt to re-align things.

Yep, do you have an air compressor? You could at least leak check the system with compressed air and soapy water. Also ensure you have flow thru the system

Do you see any powder burn/gas residue in the gas key?

 

[TxWelder35]

Teslong borescope can be used to check for gas block alignment. Like $40 on Amazon.

 

[Fx51LP308]

Yep, do you have an air compressor? You could at least leak check the system with compressed air and soapy water. Also ensure you have flow thru the system

Do you see any powder burn/gas residue in the gas key?

No. Just oil from the BCG. I did see it near the gas port, but not any further. can't remove the tube from the block that easily because it is roll pinned. I may have to look at a different block. It's not a standard one that the tube fits in easily or that might work with a standard dimpling jig. I might need the specific one from the block manufacturing.

 

[Fx51LP308]

Watch this video so instead of BANG you will get BANG BANG

Interesting. This was a Nancy Sinatra staple!

 

[GreenGO Juan]

With this you get a 3 shot burts... Bang Bang Bang

.

 

[Earnhardt]

In that case, I may as well go to my range. It's a lot closer. I just have to rent a car each time I go.

Ok wait…what?…
You have to rent a car every time you go shoot at the range?

Got no car…but you got an AR?…

 

[vinniedelpino]

Are you right on top of your neighbors?

Lawn mower in the garage with the carburetor exposed and the muffler removed. 5 gallon bucket full of sand with a steel plate at the bottom, surrounded by sand bags in a bathtub with an exhaust fan running, it doesn't take much sand to stop things. Start the lawn mower. Lawn mowers can back fire and make loud popping sounds. If you have a lawn mower that runs like shit, all the better. Hopefully you get that lawn mower running right.

I take it you're single, huh?

 

[Fx51LP308]

Ok wait…what?…
You have to rent a car every time you go shoot at the range?

Got no car…but you got an AR?…

Pretty much. I made the decision to sell my car in order to see whether or not going the car rental and Lyft/Uber route is cheaper than paying all the maintenance and fuel and insurance costs. We shall see. I'm evaluating this year and next year. If it turns out that having my own car is cheaper, I'll look at leasing one.

 

[bfoosh006]

If you are getting zero gas flow... make sure the gas tube is installed correctly.

 

[308pirate]

Yes, sadly. Definitely not some place where I can do any kind of practice or diagnostic firing.

Sadly, it's one of those lawn mowers with a "safety switch" that will kill the motor if you don't keep holding it down. I suppose I could try to "duct tape" it, but we'll see. And I'd get in trouble with the HOH for excessive Noise, anyway.

Grow some sack and do it

 

[308pirate]

In that case, I may as well go to my range. It's a lot closer. I just have to rent a car each time I go.

 

[Earnhardt]

Is the gas tube installed upside down?
Can you post pics of the barrel with the gas block/tube installed?

 

[Mike Casselton]

Ok wait…what?…
You have to rent a car every time you go shoot at the range?

Got no car…but you got an AR?…

Priorities my friend...

 

[Mike Casselton]

On a more serious note, have any of you fellers priced car insurance in the Tampa Bay area?
It's fucking outrageous!!

Arnold, buy yourself a reliable scooter and strap your toys on your back to make the trek to JTAC.

Or get this one and you have a passenger seat...

It'll cost a lot less than 6 months worth of insurance.

 

[Landon4]

You may check your trigger springs if it’s not a drop in trigger assembly, many years ago when I did my first lower I had one of the springs installed wrong and I would have to manually charge the handle to eject the brass and load another round. Once I took it apart and realized the issue and fixed it everything was fine.

 

[Ledzep]

Let's start with some reality-focused approaches to fixing your problem...

Cartridge?
Barrel length?
gas length?
Buffer weight?
Carrier?

 

[GreenGO Juan]

Let's start with some reality-focused approaches to fixing your problem...

Cartridge?
Barrel length?
gas length?
Buffer weight?
Carrier?

He said he is not seeing any signs of carbon or fouling inside the gas key.

M855 ammo was mentioned.

Sounds like the Gas Block is mis aligned or issue with the gas tube.

Regardless of Cartridge(any commercial cartridge should be close), barrel length, gas length, buffer weight. You should still see signs of gas/powder residue blowing back inside the gas key, upper receiver.

I would start at the gas block.

 

[Fx51LP308]

Is the gas tube installed upside down?
Can you post pics of the barrel with the gas block/tube installed?

No, it is installed correctly. The tube is roll pinned to the block, so it's a little difficult to take apart. But, yes, it is oriented properly. And I can blow through it and feel the air come through..... Just not at 30000 psi... If it were "upside down," I'd probably not be able to blow as hard in it as I can.

Here are some stock pics of the gas block itself.

You see the roll pin inserts here. There is one on the opposite side.

In this next shot you see the tube port as well.

Anyway, I'm confident it's installed correctly at the block. Otherwise the bend in the tube would be the wrong way (down instead of up) and wouldn't fit in the upper.

 

[Fx51LP308]

You may check your trigger springs if it’s not a drop in trigger assembly, many years ago when I did my first lower I had one of the springs installed wrong and I would have to manually charge the handle to eject the brass and load another round. Once I took it apart and realized the issue and fixed it everything was fine.

It's a Geissele Drop in. But I will check it, nonetheless. Since I don't see a lot of carbon deposits in/near the gas key, I'm going to go first with a port/block misalignment strategy.

 

[Fx51LP308]

Let's start with some reality-focused approaches to fixing your problem...

Cartridge? Federal XM855 Green Tip 62gr
Barrel length? 16.1"
gas length? Std Mid size. It definitely reaches the key.
Buffer weight? Carbine buffer set (spring & weight)
Carrier? Anderson Mfg Std. BCG.

 

[Fx51LP308]

On a more serious note, have any of you fellers priced car insurance in the Tampa Bay area?
It's fucking outrageous!!

Arnold, buy yourself a reliable scooter and strap your toys on your back to make the trek to JTAC.

Or get this one and you have a passenger seat...

View attachment 7693336


It'll cost a lot less than 6 months worth of insurance.

BEAUTIFUL! Unfotunately, it takes me 1hr to get there in a car, I can't imagine how much longer it would take in that thing.... especially with LE stopping me every few miles just to have a good laugh!

 

[hlee]

So did you ever get this issue resolved?

I mean, we're all curious as to to model of your new car...

 

[Fx51LP308]

So did you ever get this issue resolved?

I mean, we're all curious as to to model of your new car...

I have made one correction. I have attempted to re-align the block and barrel port. But I have yet to test it out. I will do that shortly.

And, if I were to buy/lease a car at this point, most likely a Toyota Highlander or 4-Runner (what I had originally). Problem being, I believe the Gen-4+ 4-runners don't have V8 available (I had a Gen3 with V-8). Just depends on what I could afford to buy/lease, at this point. A Toyota "Sequoia," while having a V8 model, is too much car and probably too expensive.

But still not there, yet. I have another 1.25 years to go before that decision. Gotta give the Lyft/Rental model a fair shot. Although I will say, my increased use of rentals to go to JTAC might influence the equation towards buy/lease.

 

[mtrmn]

Maybe check this too.

The reason you dont mount the gas block flush with the barrel shoulder

This comes up very often. Guys take there new gas block and butt it up to the shoulder of the barrel. This is incorrect unless stated differently in your gas block instructions. The reason is the proper gas block location allows for an A2 handguard endcap to be placed between the gas block and ba...

forum.308ar.com

 

[Fx51LP308]

Maybe check this too.

The reason you dont mount the gas block flush with the barrel shoulder

This comes up very often. Guys take there new gas block and butt it up to the shoulder of the barrel. This is incorrect unless stated differently in your gas block instructions. The reason is the proper gas block location allows for an A2 handguard endcap to be placed between the gas block and ba...

forum.308ar.com

That's part of the correction I just made... to ensure it's not flush against the shoulder. Ultimately, I'll probably need to get a dimple jig. I just have to ensure I get the right one. This block is a little different than others, so I may have to contact the block's manufacturer for a jig recommendation, See above for pictures of the block.

 

[hlee]

I think the recommendation for the teslong bore scope removes all doubt about gas block alignment. And, free float tubes have been a thing for decades. Gas block manufacturers have figured them out and that small gap between the block and the journal- mostly- isn’t necessary. I’ve built several ar15s and only “gapped“ the block on the first.

 

[frankxtc]

It sure is convenient to be able to test fire @ home, rather than go to the range.
Any way for you to put a silencer on it?
I shoot into a sand filled steel barrel in the garage for testing, very quiet (except for the first round “pop”, which is still not loud enough for the neighbors to hear)

 

[PlinkIt]

Worried about getting next car with a V8 while trying to not have a car to save money...

If the action doesn't cycle at all you likely have the gas system messed up.
There can be enough clearance around fitment of parts to allow small amounts of air to flow, so if you don't have ground for comparison the airflow test could be worthless.

You 100% need to for the rifle to answer questions, but just getting one to cycle at all shouldn't even be a question. You'll get less than ideal cycling easy if it's put together correctly.

Seek a local mentor or even a gunsmith to help you learn.

Take your tools with you too the range and fix it while you are there. You may even find some folks that can help while you are there

 

[Fx51LP308]

Update: Dimpling Jig received. I used it this afternoon and successfully dimpled the barrel. It went reasonably well. I was able to feel the dimple as I was attaching the gas block and I could verify that the one set screw (I did only one dimple... the one closest to the back end of the block) did seat a little lower than the other set screw, as described in one of my YT videos. Everything else checks out in terms of the gas path. I can blow well on the barrel port and feel good pressure out of the muzzle side. Same with the gas tube to the block (and I visually note the tube is properly aligned to the block). The only other issue that might happen now is if the tube and the gas key (on the BCG) are misaligned. I don't think so, but that is a possibility. Much easier to fix, I think, though. Another thing could be the buffer and spring but, again, I don't think so as I believe I got the carbine buffer/spring set. I'll measure the spring so we'll see (10.5" it's supposed to be). I will be taking my long range course this Saturday at JTAC. I had a little "prone practice" and think I got the proper eye relief to see the reticle in perfect shape. Now, I just have to work on the Parallax and then start zeroing in. After class, I will test the AR and see if my dimpling helped the situation. Hope so! I'll provide an "after action" report after class. ​

 

[Fx51LP308]

And, as promissed, here is the after-action report. The main things I do remember are



1) When in "prone" position, bring the rifle to you. Don't move your body towards the rifle. This, in re: eye relief and finding the sweet spot



2) Shoot with your dominant eye. I'm right handed and left eye dominant, so I had to learn how to do that quickly. I'm still learning. I've found, though, that doing so with the RPR isn't a huge deal as I control the bolt action. I really didn't feel so bad because my instructor is as well, and he's going blind in that Rt. eye. Thankfully, I'm not quite there just yet.



3) Don't "come off" the rifle. That is, after firing, don't lift your head but keep it on the target and as focused as possible, even when racking the bolt. An acquired skill, yes, but one worth learning.


4) Don't be scared when it goes BANG! You know when it will happen by your trigger pull. Expect it to and be ready for it



Most of today's course was about getting zeroed at 100 yds, which he got me, reasonably quickly. But I'll have a lot of practice to do as holding that RPR is a heavy weight, even when prone It was way too high. He even recommended switching from the Harris 9- 13" Bipod to the 4-6" bipod, just to keep the rifle a little lower when prone. He also recommend that I shoot from the seated position (i.e. not prone) while I learn how to relax and not let my breathing cause my body to move and compromise my aim. Also, he recommended some different "rear bags" to support the rifle better and keep still. It was a fantastic course. He will teach the 2nd "Long Range 201" course, so I'll go back to him, where we will learn how to go beyond 100 yards, how to "true" a firing position. how to use a Ballistics computer, etc. (he called my Mil-Dot master "old school" and strongly recommended a Kestrel). I might get there before I go to that course, but we'll see. Also, if anything changes in your formula (different ammo, different positions) you need to re-zero. Always carry a torque wrench with you to make adjustments and keep everything in proper torque.

Towards the end of the day, we also did some "aiming drills" that were designed to build confidence in being able to get on the weapon aim and fire, then get off the weapon, relax and then get right back on. We also shot at different sized targets (progressively smaller) to promote better accuracy. It was working. And, of course, in between firing live rounds, we had to "dry fire" 5 times to keep in good shape. All, very good practice.


Now, for the AR, which I got to shoot again after class. Well, good news/bad news. Good news is, my changes to the gas system seem to work to the point of being able to make the BCG move back and eject the spent brass. I can see it eject. And, since I'm shooting left eye, sometimes I can even feel it!

Bad news is, it may not be enough gas, as I"m getting consistent failures to feed. A manual cycling often does resolve the issue, but I'm also seeing quite a bit of jamming, which I didn't see earlier. In fact, it's so jammed right now, and the only thing I can do is disassemble the upper and pull out the BCG to clear the jam. I can't even remove the mag as a result of this jam.


So, I'm now looking at a couple of things. 1) Is there enough gas not only to eject the round but, also, does the BCG go back far enough to allow a new round to feed? Is the BCG itself somehow not alligned well enough causing the feeds to miss? Could it also be a buffer/spring issue and do I have the right buffer/spring for the task at hand? Although, I'm not sure if I should go heavier or lighter. We'll see.


Anyway, that's the range report. Overall, a fantastic day on the long range. We'll fix the AR. At least I know it's getting gas, now. How much, TBD.

 

[mtrmn]

When dealing with these type of issues with ARs, only put 1 round in the mag. If gas is right it should eject and lock open on the empty mag. If it does not lock, it ain't getting enough gas IF all else is good. This method will prevent many of the seemingly hopeless jams caused by the extra rounds in the mag until you get it figured out.

 

[Earnhardt]

Here's what I would do....

-remove the gas block and tube
- inspect the gas port on the barrel, to ensure the port is completely open and unobstructed
-use a pin gage to ensure the gas port is the correct size
-if the gas pot is too small, use the correctly sized Cobalt drill bit, and open up the gas port (to the correct size) with a wood dowel, or non metal cleaning rod in the barrel while opening/cleaning out the gas port. That will prevent the Cobalt drill bit from hitting the opposite size of the barrel as the gas port

-since non-Adj gas blocks and gas tubes are cheap, I'd install a new/different gas tube and gas block

re-try...

 

[Fx51LP308]

When dealing with these type of issues with ARs, only put 1 round in the mag. If gas is right it should eject and lock open on the empty mag. If it does not lock, it ain't getting enough gas IF all else is good. This method will prevent many of the seemingly hopeless jams caused by the extra rounds in the mag until you get it figured out.

Or, there's something wrong with the lock. And that might be the case for me, but I will definitely try the "one round" method if I haven't fixed anything by then. I agree it will help by not creating any jams, but that problem will have to be resolved at some point (assuming it's not related to gas).

 

[Fx51LP308]

Here's what I would do....

-remove the gas block and tube
- inspect the gas port on the barrel, to ensure the port is completely open and unobstructed

It was when i checked it this last time. I was able to get good pressure from the barrel port.

-use a pin gage to ensure the gas port is the correct size

What size should that be? I know the barrel port is slightly smaller than the Gas block/tube port combination. I'm not sure how big the barrel port should be.

-if the gas pot is too small, use the correctly sized Cobalt drill bit, and open up the gas port (to the correct size) with a wood dowel, or non metal cleaning rod

That might be an option.

-since non-Adj gas blocks and gas tubes are cheap, I'd install a new/different gas tube and gas block

re-try...

Unfortunately, I have to use this particular gas block as no other will fit against the HG I'm using. I will be checking out your barrel port scenario, nonetheless. It might need a bit of an increase.

 

[mtrmn]

MOST 16" barrels with mid length gas seem to have gas port diameters between 0.070"-0.080".

 

[Fx51LP308]

MOST 16" barrels with mid length gas seem to have gas port diameters between 0.070"-0.080".

Thanks! I'll check it out.

 

[mtrmn]

ALSO! If you don't have the pin gages or a set of number drill bits to measure the gas port, you can go to ACE hardware or anyplace that sells welding supplies and get a folding set of tip cleaners for cutting torches. Find the one that's closest in size and measure it with calipers or a micrometer.

If your port is on the low end of the specs, I'd go ahead and drill it out to the high end and then use adjustable gas block to tune from there. Get a wood dowel that fits the bore while you're at the hardware store anyway. Use it to prevent damaging the opposite side of the barrel while drilling.

 

[Fx51LP308]

The barrel port appears to be withn the .07-.08 range. In fact, having just dissassembled the upper/lower and cleared the jam I had, I honestly believe the gas system is now in good working order. I think it's time to move onto the upper/BCG, which could be "binding."

I cleared the jam very carefully. As it turned out, there was a live round "in battery" but not fired. Another was right behind it with the tip jamming against the "in battery" round. I had to separate the upper & lower, but even that wouldn't clear the jam. I was,
finally able to remove the mag (one round was jammed between the mag and upper. I had to push back the BCG, manually with pliers (which it did) and use a pair of needle nose pliers to clear the live round jammed up against the "in battery" round. Once I did that, the BCG
itself also went into battery. I pulled the charging handle again and the "in battery" round did eject, clearing the rifle.

Thinking it might have something to do with the tension the rounds are in when loaded into my PMAG-30s, I took an empty one and loaded 10 5.56 snap caps into it. I charged one, which went into battery. Now, the charging handle is now jammed. It won't pull back. I got the PMAG
out, but I can't pull the charging handle at all to eject the snap cap.

At this point, I'm considering replacing either the BCG or replacing the entire upper. Something is JDFR with it. You were right in that I don't think it's the buffer/spring assembly. I think it might very well be that "BCG binding" to which you refer. But what to do about it. I also need to ensure
that there isn't an issue with the barrel itself not allowing the spent round to eject. I think they are ejecting properly (the jammed round and the "in battery" round certainly did). But I think something is causing the BCG to bind. Also, I've noticed the mag release on the lower is not working well either. It doesn't lock back when the last round is ejected. But that's a different issue for another time.

I also checked for a gas leak when I had the upper/lower separated. I think the gas system is OK as the key goes right into the tube when in battery. I think my gas issues are resolved now. I just have to move on to the upper/BCG issues.

​

 

[hlee]

Possible that the chamber is fubar and causing extra drag when extracting the spent cartridge. Does the bcg jam in the barrel without a cartridge? There is no reason that a snap cap should cause the action to jam.

My 22 Grendel was sluggish for the first few mags, but eventually smoothed out. My hypothesis is that a minor chamber imperfection was dragging on the spent cartridges, but polished out with use. Possible that the bolt lugs needed to wear in too.

Can you manually lock back the bcg? Does it lock on the bolt or the nose of the carrier?