When I shoot the last round in a mag, the bolt doesn’t lock to the rear. This happened a few months back and I sent it back to the manufacturer for repair. I bought from a local place (didn’t read this forum at the time) and they had it “fixed” in a couple days. They told me it has to do with the amount of gas coming back from the gas port.
Is this A: A normal problem, B: Something that can be fixed forever by tuning the gas port to a certain point, C: Something I can fix myself?
Did this shop explain exactly what they did to correct the short stroking the first time? I would look there for starters. After-market gas block slipping perhaps?
Embarrassed to say it, but I didn’t even ask. They were so highly recommended that I just assumed it wouldn’t happen again. I realize now how foolish that assumption was.
I’m at work right now. I don’t know what markings are on the buffer yet but ill check when I get home.
I’m assuming by the lack of immediate fix that this is an unusual problem? Anyone ever heard of adjusting the amount of gas coming back from the gas block or were they just feeding me a line?
Short stroking is one of the more common AR problems. The difficulty is troubleshooting due to the variety of probable causes. Troubleshooting over the internet doesn’t make it any easier…LOL.
Since you’ve had similar problems in the past with this weapon, and took it in for repairs where you were told the defect was in the gas system, now that the weapon is exhibiting similar symptoms, that is where I would personally start.
But since we don’t know what corrective action the shop took, we are back to square one.
What type of gas block is on the weapon? If it is of the set-screw variety, could the block have shifted, partially blocking the barrels gas port?
Going back through the gas system from there:
Is there gas leakage where the gas tube enters the FSB or gas block?
Is the gas tube worn, causing excessive clearance between the gas tube and carrier key?
Bolt carrier key loose?
Bolt gas rings worn?
Another not too often occurrence would be excessive carbon build-up in the gas tube.
ETA: These are of course a few probable causes on the gas system side. There could be a mechanical problem as well.
A few of those probable causes might be:
Lack of lube.
Interior of the weapon excessively dirty or fouled.
Obstruction in the receiver extension limiting carrier movement during firing.
Or weak magazine springs that are strong enough to lock the bolt back when you’re doing it manually, but not strong(fast) enough to get up in time to lock it back when the BCG is going much faster.
When you say this is the “second time my gun has done this”. Do you mean that it actually has failed to lock back two different times?
On a gun that works, the number one cause for a failure to lock the BCG back to be mag related.
Does it happen with a particular magazine(s)? If I get a failure to lockback, I immediately mark the mag, then try 2-rnds to see if it happens again. If it does, I clean the mag, then try 2-rnds again. Third time to fail, I mark it as a Range Mag and never count on it again.
Again, on a gun that works, the next place to look is the ammo.
Did it happen with other ammo you have used. Some of the WM 100-rnd packs are 50g bullets. Does it work OK with heavier ammo?
If this doesn’t narrow the cause down, then I’d look at springs and the gas system but I’d be surprised if it is not a mag or the ammo, especially if the gun generally “works”.
The gun fires and cycles properly. The only thing not.functioning seems to be the bolt locking to the rear after the last round in a mag. I doubt its a mag or ammo problem as I was using a couple types of ammo and it occurred with 8 different pmags. I could see a problem with one but all 8 at the same time seems unlikely.
By “second time” I mean that the gun had the same problem a few months back. Many different occasions the bolt would not hold open on an empty mag. I sent it off to be repaired, got it back and.the.problem was fixed.
Sorry for.all the weird periods. Autotype on my cell phone.
As far as it being dirty or obstructed, I doubt it. I break down and meticulously clean all my guns every three weeks regardless of hoe.much I’ve shot them. I hadn’t shot the.rifle in between the last two cleanings though. It may sound silly but could not firing it be bad? Obviously a lack of.shooting is bad for.me, but I’m talking about something regarding the workings of the gun. Should I be cleaning it less, I guess?
Y’all thanks so much for the.help. I wish I hadhalf the knowledge and experience most of y’all do.
I bought a case of pmc at a gun show and was running that when it happened the first time. I still had some left over, already loaded in.mags. we were shooting that as well as the Walmart federal. However it was happening with all my mags, which would have been loaded with some of each. I mean, a mag of pmc and a mag of federal.
Hopefully its something that will be fixable. The gun has a lifetime warranty on parts and labor. I’m less concerned about that than I am it becoming a larger problem if I need it in a dangerous situation.
Contact this company- I went to their website and found very little info. Ask them what size gas port your rifle has. I have a feeling that they are going to stall you or not give you a straight answer.
Check the things we have told you to check. Gas block, FSB, etc… for gas leakage. Make sure that the bolt carrier key is tight and the screws are staked. Also, check your gas rings and make sure they are tight.
My guess is that you have a standard buffer. This can be identified by looking at the face of the buffer. It will either be plain, or have an “H”, “H2”, etc…I would get an “H” buffer.
Try and get some true 5.56 (not .223) ammo and then take all of it to the range. Load all of your mags with one round only and shoot them. See if the bolt locks back. Make sure that your BCG is wet. Not just a few drops of oil.
Some other things you may want to find out are whether or not you have an auto BCG or a semi-auto or one of the other variants.
Another thing to check is a stiff/rough operating bolt catch. I’ve had to trim a few bolt catch springs due to excessive tension. It really doesn’t take much.
Excessive bolt speed is something that gave me a problem before when combined with a stiff bolt catch. The bolt would outrun the mag follower and not lock back.
I experienced the bolt not locking back on the last round due to weak springs. You can test this by putting one round on each of your magazines, shoot them all, and note down which magazines caused the bolt not to lock back. I did this test and found 3 bad magazines that I owned. They were 10/30 Pmags… go figure.
I absolutely concur with Iraqgunz.
The only thing I would do to speed up the process is to find some NATO pressure ammo (Fed XM193 or M855 is the easiest route) and do a mag-check as explained by a few posters above.
Some pics of critical parts of the gun may also help:
Close up of gas block
Close up of carrier key
Face of buffer
Side profile of bolt carrier group
Bolt/gas rings
Bolt catch with empty magazine inserted with the upper removed (the actual catch, not the bolt release)
Another one I experienced was a broken gas key bolt. The front bolt snapped, but you couldn’t really tell. The head was tightly staked in and the rear held the key tight. But there was a gap under the key that leaked gas.