Can anyone offer suggestions on ftf for an AR in 6.8 based on a picture? JP rcvr set, spring and carrier, ARP barrel and bolt, headspace checks, brand new PRI 25rnd mag. Happened same way with this SB ball and 120g sst reloads. Only about 150rnds total in the rifle, didn't happen with my other 15rnd PRI mag. I didn't get time to do much experimenting, was shooting at some pigs the other evening and left for work the next day. I did clean it and it happened a second time so I snapped this pic. Normally I'd get some ammo and my other rifle and figure it out, but trying to save my ammo. I suspect the mag, but it's PRI? Never had an issue before, but this one is my first 25rnd from them. No smiths near me that I know of, appreciate any help.
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Help diagnose failure to feed
- Thread starter wild_musk
- Start date
- Nov 3, 2010
- 9,696
- 20,373
Looking at it from a "cost" point of view... Adjusting the lips on the mag could solve the issue... Don't over think the issue...
.......didn't happen with my other 15rnd PRI mag.........
In this case, just look at what you changed.
.......didn't happen with my other 15rnd PRI mag.........
In this case, just look at what you changed.
I should have included it's a suppressed 10.5" barrel. JP adjustable gas and a heavy buffer (I don't remember exactly, h2 if I had to guess). I have to experiment more with those components, I feel the same way. Although it functions, the gas is wide open to get the bolt to lock. Brass kicks out between 1-3 o'clock. The rifle is mostly borrowed parts from a diff build while I wait to buy the final parts. Most will be same or similar, but I want to get the gas system tuned better. I didn't consider this could also lead to ftf issues, thanks.
I didn't see any issue with the case heads but will look more closely.
I hate to think it's that magazine, just bought 4 of them because they're "the best" and I don't like jacking with feed issues. I'm gonna keep on wishing for good shit to happen to me, but prob need to look at that mag like you suggest...thanks
I'll oversimplify the situation... It's just mechanics. Don't even think of it as a weapon. Think of an old metal garbage can with a bent up lid. Just a matter of movin' some metal around to make the lid stay on. Probably just need to move a little metal on one mag. Once you get that one to cycle just do the same to the others.
Remain calm, don't get frustrated. Lots of help online and here on the forum.
Lol thanks...a garbage can is a good descriptor of how I felt about it the other evening. 25 or so hogs mixed in with the cows, brand new rifle, first real world test.... boom click. So on top of figuring this issue out I have to practice ftf drills, because I sucked. The damn women giving me crap about letting them all get away.
I have walked many miles in your shoes... I have explained to many 1st year apprentices when they see two locomotives speeding towards each other it's best to calmly take a few steps back and watch the train wreck. You just need a good healer to sort the livestock. Weapons and women are no different. You will be OK.
When I have had the failure to feed and crushed brass like you have it has been because of under gassing and when setting an adj gasblock. Considering your adj gas block is open completely you can’t add gas so lighten the mass. Throw in a carbine buffer and see if it’s better. I’m relatively new to AR stuff but this has been my experience.
Thanks, setting up a gas system in a new rifle is always a challenge for me. I'll try a lighter buffer also. I do have one of the jp captured springs with the adjustable weight system. I was hoping to avoid using it unless nothing else works.
Let's go through some general concepts. Keep in mind I don't have experience with your exact cartridge.
"Dwell time". If you are not familiar with that term google it. Basically it is the amount of time from when the bullet passes the gas port until it exits the muzzle.
Once that exit happens then pressure falls off substantially and almost instantly.
If there is say two inches of barrel past your gas port the dwell time will be X. If you had six inches of barrel past the gas port the dwell time would be Y.
It's during that short window where the operation of the gun is reliant on gas to function.
There is a really complicated song and dance about HOW all that works together.
In your case you are increasing the total volume of gas to the system, but the duration is short.
In either case definitely try a much heavier buffer, like an H3 or H4. You have a huge blast of gas over a really short duration so you need to fight it back some with a stronger buffer spring or a heavier buffer. I have guns with both a heavier spring as well as really heavy buffers so don't be scared to move up.
From what I'm seeing use a heavy spring and then move up at least two weights from where you are at now. It shouldn't be any more than say $75 bucks all in.
There are other things you can do such as playing with the gas port size (do your homework before you do it though) or playing around with reloading components. That said I would not touch anything else right now until you try the cheap and easy way first.
Obviously, you are under-gassed. This may be your sole issue, or it may be one of several, but even then, it's your biggest issue. Go to a lighter spring AND a lighter buffer.
Why? It's the cat's meow. Better than anything else on the market by a good margin.
And beware, when you ask advice on ARs, everybody and their mother jump on and give you the advice to go to a heavy, h2, h3 buffer and heavy springs without even understanding your problem. They only say that because it's repeated as the popular "cure-all" for all AR problems these days by people who don't have a basic understand of how a BCG mass and spring rate oppose gas volume (Let's see. . . gas block on max setting and barely locking back. . . yep, you need to add more mass to the carrier to make it even harder to lock back, lol).
You must have missed what he said:
Only about 150rnds total in the rifle, didn't happen with my other 15rnd PRI mag.
What's your take on the bolt locking back on full gas but not on any reduced setting?
I would start with the basic, simple things like giving it a good cleaning and apply some really good lube. It is fairly new so it may just need to run in.
ARP barrels are usually gassed conservatively. It used to state on his website that they are ported to work with a carbine buffer and he does have info on the site about troubleshooting short stroking. Harrison is usually very helpful to his customers. Shoot him an email or give him a call.
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Appreciate all the responses. Lots to think about and test when I get home. Fortunately I have several buffers, plus the adjustable JP captured spring, and a standard captured spring...and I think I should make sure that adj gas block is on right. I knew this might be tossing a crap grenade, but I appreciate all the responses.
I did email Harrison also, but wanted to get multi opinions. There's just so many variables once you start changing calibers and barrel lengths....easy to order a bunch of parts, even quality parts, but none of this is standard. I think I have what I need to go figure some stuff out though.
I did email Harrison also, but wanted to get multi opinions. There's just so many variables once you start changing calibers and barrel lengths....easy to order a bunch of parts, even quality parts, but none of this is standard. I think I have what I need to go figure some stuff out though.
Please post a response after you do some testing and let us know the results.
Did some testing today with different buffer weights and two different springs in the JP silent captured spring. More to follow, but here's what I got. I only tested to see if I could get the bolt to lock back and note brass ejection for different buffer and spring weights. I want to get the system operating correct before I address the FTF issue I initially asked about.
So I had 2 JP SCPs to work with and one XS spring pack I bought a couple years ago for another build - the XS springs are the smaller ones to the left in the pic. I weighed out the buffer system parts individually.
The SCP comes with tungsten and steel weights and a standard "AR-85" spring. Tungsten weighs approx double steel, and that's Tungsten in the pic at 1.5oz. The system has 3 weights total. The XS springs were .2 oz lighter than the standard spring. They are lighter functioning springs for when your bolt doesn't lock back with full gas. I machined an extra steel weight also, and it was very close to the JP one.
Here are a couple of useful things I pulled off the interwebs...they helped guide all my incorrect suppositions about the gas system.
Buffer weights I pulled online...
Additionally, the JP instructions are also useful, I guess. I plan to read them. Eventually.
Test 1: 2x Steel/1x Tung, XS-100 white/black spring...approx 3 oz
--locks back with minimal gas, ejection at 2 O'clock (I started with the gas almost closed, then opened it up slightly to get to this result over a couple tries...I could have opened up more to see how/if the brass ejection changed, but didn't want to wear myself out.)
Test 2: 2x Tungsten/1x Steel, XS-100 white/black spring (3.7oz)
--locks back with approx 1/2 turn more gas, ejection at 2:30-3 O'clock
Test 3: 1x Tung/2x Steel (3oz), standard JP AR 15-85 (black) spring.
--locks back with approx 1/2 gas, ejection at 3:30
Test 4: 2x Tungsten/1x Steel (3.7 oz), standard JP AR 15-85 (black) spring.
This was the combo I had at full gas initially, so I expected it to fail. But it locked back, same gas setting as test #3. Brass at 3 o'clock
Test 5: 3x Tungsten, std spring (4.5oz). Locked back, same gas as test 3 &4. Brass at 3:30.
So I think I did something wrong in the initial adjustment of the gas block to have it at full in order to cycle. I mean, that's confusing because you start at closed and open until it cycles, then more until it locks the bolt back...and I had to take it all the way open to get there, same ammo, but today it seemed to function normally with any weight setup.
Now that I know it will function basically at all these combinations, I need to choose which one is "best" and fine tune it. JP says I should be able to feel if the spring is fully compressing or not, hitting the back end, but so far I didn't notice. And, It needs to be checked suppressed, because I already forgot my ear pro once today, and I'm done with that bullcrap.
So I had 2 JP SCPs to work with and one XS spring pack I bought a couple years ago for another build - the XS springs are the smaller ones to the left in the pic. I weighed out the buffer system parts individually.
The SCP comes with tungsten and steel weights and a standard "AR-85" spring. Tungsten weighs approx double steel, and that's Tungsten in the pic at 1.5oz. The system has 3 weights total. The XS springs were .2 oz lighter than the standard spring. They are lighter functioning springs for when your bolt doesn't lock back with full gas. I machined an extra steel weight also, and it was very close to the JP one.
Here are a couple of useful things I pulled off the interwebs...they helped guide all my incorrect suppositions about the gas system.
Buffer weights I pulled online...
Additionally, the JP instructions are also useful, I guess. I plan to read them. Eventually.
Test 1: 2x Steel/1x Tung, XS-100 white/black spring...approx 3 oz
--locks back with minimal gas, ejection at 2 O'clock (I started with the gas almost closed, then opened it up slightly to get to this result over a couple tries...I could have opened up more to see how/if the brass ejection changed, but didn't want to wear myself out.)
Test 2: 2x Tungsten/1x Steel, XS-100 white/black spring (3.7oz)
--locks back with approx 1/2 turn more gas, ejection at 2:30-3 O'clock
Test 3: 1x Tung/2x Steel (3oz), standard JP AR 15-85 (black) spring.
--locks back with approx 1/2 gas, ejection at 3:30
Test 4: 2x Tungsten/1x Steel (3.7 oz), standard JP AR 15-85 (black) spring.
This was the combo I had at full gas initially, so I expected it to fail. But it locked back, same gas setting as test #3. Brass at 3 o'clock
Test 5: 3x Tungsten, std spring (4.5oz). Locked back, same gas as test 3 &4. Brass at 3:30.
So I think I did something wrong in the initial adjustment of the gas block to have it at full in order to cycle. I mean, that's confusing because you start at closed and open until it cycles, then more until it locks the bolt back...and I had to take it all the way open to get there, same ammo, but today it seemed to function normally with any weight setup.
Now that I know it will function basically at all these combinations, I need to choose which one is "best" and fine tune it. JP says I should be able to feel if the spring is fully compressing or not, hitting the back end, but so far I didn't notice. And, It needs to be checked suppressed, because I already forgot my ear pro once today, and I'm done with that bullcrap.
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As far as the FTF, it's the magazine...
I called PRI, they are taking care of it. It's a small difference, but see how the round tip sits lower in the left magazine? That's the problem. It should be angled slightly up like the one on the right.
On the plus side, I have more confidence in my gas system setup. I went back and adjusted with the suppressor on, but found I actually couldn't change much. Attempts to reduce gas even 1/2 turn resulted in the rifle barely spitting out the brass and no lock back.
I called PRI, they are taking care of it. It's a small difference, but see how the round tip sits lower in the left magazine? That's the problem. It should be angled slightly up like the one on the right.
On the plus side, I have more confidence in my gas system setup. I went back and adjusted with the suppressor on, but found I actually couldn't change much. Attempts to reduce gas even 1/2 turn resulted in the rifle barely spitting out the brass and no lock back.
This kind of test is very useful, if for nothing else but sheer learning. Not everyone has that many buffer combinations available. In any case it's great to do isolation testing. If it truly is the mag causing some kind of weird binding thing where the brass gets crushed by the feed lips then it's good to know that.
Based on your writing, I would think #5 might be a better option, but then again I wasn't there. If brass is ejecting forward that's not the best so the first few are out I would think. You probably could go even heavier and see how that works if that's an option.
Based on your tests, what do you think seemed to offer the best of all worlds in relation to buffer weights/ejection/reliability/and overall function? If it's not #5 that's totally fine, I am just wondering YOUR impressions.
Also how far was it flinging the brass? Was it sending it to the next county over, or was it barely clearing the ejection port?
Keep in mind, depending on a variety of circumstances there is always a chance that excessive bolt speed (over gassing) can magnify problems in a magazine.
By all means let them take care of the problem if you determine there is one. That said in one gun with normal carrier speed the problemed mag might work just fine, but in another gun with a faster carrier speed it might not.
Regardless I am glad you did this experiment. It's definitely helping to isolate the problem.
Like I said above, if it's a possibility, I would try to go even heavier on the weight and see. Just as an experiment that might yield interesting results.
Lol, I think the best option is my other rifle...16" mid with a standard gas block and 3.7oz buffer. It's noticeably smoother than the short configuration. The tradeoff for short vs smooth shooting was the biggest thing I learned today, at least in my setups.
I could shoot each weight again and actually fine tune the block for each one. I didn't change the gas at all for the last 3 tests, and it seemed optimal for the heaviest buffer. I guess I could tune it down for the lighter buffer combos, but maybe it's not worth the ammo. At least not until I understand the tradeoffs better.
I could shoot each weight again and actually fine tune the block for each one. I didn't change the gas at all for the last 3 tests, and it seemed optimal for the heaviest buffer. I guess I could tune it down for the lighter buffer combos, but maybe it's not worth the ammo. At least not until I understand the tradeoffs better.
A cheap but messy test, slop your bolt carrier down with some heavy grease to slow it down. See what it does.
They were all slinging out 4-5 feet at the settings that allowed lockback...I didn't measure it, just normal. I dialed the gas back with the suppressor initially and one hung up, the next barely dropped out.
I don't think I can, at least with the JP spring. 3x Tungsten weights maxes it out.
I can do that...I just switched to weapon shield from rydol after the first malfunctions and cleaning it.
Seems like, at least with the short barrel, increasing the buffer weight slowed the carrier and ejected the brass more towards 4 o'clock. I'm not sure changing the gas volume affected brass orientation. Then I came across some web wisdom that contradicts all of this, and maybe that's more correct. I'm not a scientist. I want to shoot some more and try to feel a difference in follow up shots, recoil, etc with lighter and heavier buffer weights, since it seems like any of them basically function.
One thing I noticed that you never mentioned. What is the brand of the AGB? Is it a Superlative Arms?
What did the dented brass match? The lugs of the barrel extension or the lugs on the bolt? Curious if the angle had the bolt run over the top of the cartridge rather than pushing it up the ramp.
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The AGB is a JP A2 model. The brass is hitting the feed ramp while the back end is still held down by the feed lips of the mag. I loaded a second one yesterday and noticed anything over 20 rounds or so will usually not feed the first round, especially not if the bolt release is used to charge it. Less than 10-15 rounds in the mag and the follower floats enough that they mostly function.
I asked PRI how they fix it and was told they adjust the angle slightly. I wouldn't mind trying it myself, but they have the tooling, and I have some pliers ha. But I think I anyone could do it if they had to.
I think Brownells sells a feed lip adjusting tool made by HS Precision. It's worth owning one although on a brand new mag, yeah, let them deal with it.
That's good to know, I will look at that with my next order.
Also, fwiw, brass kicks out at 2 o'clock with the suppressor attached and no other changes. Seems like that is normal, or in line with what others have reported.
The standard fixed-gas AR system works great for most full-power factory ammo in 5.56. But they are typically over-gassed, and can have reliability problems when going to other chamberings, gas system lengths, suppressed, and other modifications. So we attempt to restore reliability or make a smoother shooting rifle by better balancing the gas and reciprocating mass. And there's more than one way to skin that cat.
The two basic approaches to balancing the gas and reciprocating mass are:
1. Increase reciprocating mass (buffer and BCG). This is the oldest and most popular approach and was originally developed for guns with fixed gas blocks to combat the effects of the typically over-gassed standard system (balance the excess gas by increasing reciprocating mass). This slows down the carrier because the same amount of gas now has to move more mass, which has greater inertia and thus, a slower rate of acceleration in both directions.
The upside is a smoother shooting and more reliable gun.
The downside is more felt recoil and rifle upset due to more reciprocating mass.
2. Decrease reciprocating mass AND gas. This is the newer approach and requires an adjustable gas block (AGB). With an AGB there is now no need to have excess gas in the first place, so there is no need to suffer the increased felt recoil and rifle upset of increased reciprocating mass. The biggest benefit to this approach is that you can go to a very lightweight reciprocating mass for less felt recoil and rifle upset and just balance it with the AGB. Apparently, JP Enterprises also sees merit to this approach, as they pioneered the highly praised LMOS (low mass operating system) to reduce recoil and make a smoother shooting rifle.
The upside is a smoother shooting and more reliable gun AND reduced felt recoil and rifle upset.
The downside is it may require a gas block with finer adjustability settings to get the perfect balance (which is one reason a lot of people prefer SLR's gas blocks).
Also, people go to heavier springs to try to solve these problems, but that's often a mistake, as the higher the spring rate, the harder the spring slams the BCG back into battery. There can be a time and place for that, but it's typically best to adjust reciprocating mass and gas first and see if that fixes your problems.
I recently built an 8" barreled, suppressed AR with a lightweight BCG, JP SCS with standard spring and standard buffer, and SLR gas block and it's the smoothest shooting AR I've ever been behind. Recoil and rifle upset between shots are so minimal it's silly.
The two basic approaches to balancing the gas and reciprocating mass are:
1. Increase reciprocating mass (buffer and BCG). This is the oldest and most popular approach and was originally developed for guns with fixed gas blocks to combat the effects of the typically over-gassed standard system (balance the excess gas by increasing reciprocating mass). This slows down the carrier because the same amount of gas now has to move more mass, which has greater inertia and thus, a slower rate of acceleration in both directions.
The upside is a smoother shooting and more reliable gun.
The downside is more felt recoil and rifle upset due to more reciprocating mass.
2. Decrease reciprocating mass AND gas. This is the newer approach and requires an adjustable gas block (AGB). With an AGB there is now no need to have excess gas in the first place, so there is no need to suffer the increased felt recoil and rifle upset of increased reciprocating mass. The biggest benefit to this approach is that you can go to a very lightweight reciprocating mass for less felt recoil and rifle upset and just balance it with the AGB. Apparently, JP Enterprises also sees merit to this approach, as they pioneered the highly praised LMOS (low mass operating system) to reduce recoil and make a smoother shooting rifle.
The upside is a smoother shooting and more reliable gun AND reduced felt recoil and rifle upset.
The downside is it may require a gas block with finer adjustability settings to get the perfect balance (which is one reason a lot of people prefer SLR's gas blocks).
Also, people go to heavier springs to try to solve these problems, but that's often a mistake, as the higher the spring rate, the harder the spring slams the BCG back into battery. There can be a time and place for that, but it's typically best to adjust reciprocating mass and gas first and see if that fixes your problems.
I recently built an 8" barreled, suppressed AR with a lightweight BCG, JP SCS with standard spring and standard buffer, and SLR gas block and it's the smoothest shooting AR I've ever been behind. Recoil and rifle upset between shots are so minimal it's silly.
It's over gassed, period.
You'll know for sure when you sheer off your extractor claw. Ask me how I know.
As was mentioned, there's a delicate balance with all these components. If turning the gas down at all induces a stoppage I'd be looking at a lighter buffer and a lighter spring combo.
You need to dial that back and use the chart already posted. Ideally it should be ejecting between 3-5:00
If it's spitting brass at 1:30 (and I imagine 10-15ft forward from your firing position) you're beating the fuck out of that gun.
Can you cobble together a gun and have it work, yes, does that mean it's correct... not necessarily. Good luck
So, initially, yes.. brass position moved from around 2 o'clock with the lightest combo I have (3oz buffer and the JP XS spring). When I went to the standard spring and heaviest buffer, brass moved to 4 o'clock. The brass kicks out 4_5 feet away at all of those settings, less for gas settings that did not lock the bolt back. Appreciate the advice, I will definitely keep it in mind going forward.
I know about the JP LMOS, but it seemed geared toward competition guns. I still need to purchase a BCG for this build and give these parts back to the rifle I stole them from, so I think I'll try it out. The lighter XS spring set came with 3 weights, all significantly lighter than the standard captured spring. I'll have to ask JP if that combination is ok or maybe someone already has a similar setup.
Just watch that ejection pattern, it's a great indicator of gas pressure. If you're around 4:00 I'd leave it alone.
If you're at 1:30 you've got work to do because you're beating up the gun.
The components work opposite each other. More spring pressure and buffer mass take more gas's to drive them, opposite is also true.
I was under the impression you were ejecting at 1:30 and if you tried to close the gas down to move the ejection pattern back you wound up with jams. Hence my suggestion to think less spring pressure and buffer weight, that needs less gas to drive, so you could dial it back and still have it function.
It's a dance sometimes getting all this to work properly
If you're at 1:30 you've got work to do because you're beating up the gun.
The components work opposite each other. More spring pressure and buffer mass take more gas's to drive them, opposite is also true.
I was under the impression you were ejecting at 1:30 and if you tried to close the gas down to move the ejection pattern back you wound up with jams. Hence my suggestion to think less spring pressure and buffer weight, that needs less gas to drive, so you could dial it back and still have it function.
It's a dance sometimes getting all this to work properly
It seems to be working fine as is, which is good because I can always use this combination of parts. I cannibalized the parts for this build from another rifle to see how everything worked before I bought yet another BCG or silent captured spring....
