I searched around and I am having trouble finding a definitive cause or solution to the issue I am experiencing. If I missed something, then I apologize for creating a superfluous thread. Feel free to link me to existing threads if they apply.
The issue:
I built the rifle a couple of months ago and have since put about 500 rounds through it. The first 400 or so went through flawlessly. More recently, the bolt seems to cycle, eject the casing, grab a new round, and then get stuck about a half of an inch short of closed. It started doing it once or twice in each 20-rd mag that I put through, but now it’s doing it on almost every round. If manually pull the charging handle back a bit (not so far as cause a double-feed) and then sling-shot it, it chambers the round the rest of the way and goes all the way into battery just fine.
If I lock the bolt back and manually place a round into the chamber, it gets very stiff and difficult to push in for the last 3/8" or so. It’s almost as if the casing is just too large, or the chamber bore is just too small. It didn’t seem to have any issue with this in first few hundred rounds through it.
The rifle has been cleaned after every hundred rounds or so, and re-oiled before its next use. Any advice or suggestions on what could be causing this, or how to fix it?
The rifle:
Lower:
PSA Stripped Lower Receiver
PSA Magpul Lower Parts Kit
Upper:
BCM 16" Carbine-Length Upper Receiver Assembly
BCM Gunfighter Mod 4 Charging Handle
BCM Bolt Carrier Group
Did you change brands or types of ammo? Did you really scrub the chamber or just swab it and oil? I’ve seen this happen on other rifles and it’s always been something in the chamber.
20-rd PMAGs with 10 rounds loaded in each, so I don’t think it’s a magazine issue. As I said, the bolt is grabbing a new round and carrying it into the chamber just fine. The problem is that it can’t seem to get it the last half-inch or so.
I have been using mostly cheap range ammo, such as Brown Bear and PMC X-Tac, but I’ve also tried it with mid-range stuff such as Winchester 5.56 and Federal AE XM193. This is my first AR, so it’s possible that I am doing something wrong with the cleaning, but I’m not new to rifles in general. The 3 times I have cleaned it, I have sprayed the chamber/barrel with aerosol CLP, then wire-brushed, then run patches through with solvent, then cleaner, then a final lube. I also spray and brush the BCG. Should I be doing more during a simple post-range cleaning?
See above.
In an effort to diagnose the issue, I attempted to seat a round into the chamber by hand. (Meaning place the nose in the chamber, and push from the rear with my finger.) The first half of the round went in fine, but when it got to the last half-inch or so, it got noticeably stiffer. Again, this is my first AR, so I still have a lot to learn. Perhaps this tight fit is normal, but it seems to be what is causing the problem.
The bolt releases forward, grabs a round on the way, begins to chamber it, the round goes in straight, and then the bolt gets stuck about a half-inch shy of closed.
If I understand short-stroking correctly, isn’t that when the bolt doesn’t travel far enough to the rear and is unable to catch the next round? This is not the problem. The bolt grabs the next round just fine, it just isn’t able to drive it all the way into the chamber.
See above. Perhaps it’s an issue with the cheap ammo, but as I said before, I had no problems with any of it in the beginning. It ran just fine for the first 300-400 rounds on the cheap stuff. Also, please correct me if I am wrong, but is it not safe to assume that a BCM rifle with all mil-spec components should be able to chew through just about any ammo without problem?
As mentioned above, I am using 20-rd PMAGs which are typically loaded with 10 rounds each.
Yes, by “build” I mean that I bought a stripped lower, installed all the components, then bought a completely assembled BCM upper and mated the two together. I also bought an assembled bolt carrier group from BCM and dropped that into the upper as well.
Measure your buffer spring to make sure it is within tollerance. Then check the installation of the botl catch to see if it is binding or not.
Then I would check for burrs in the chamber. Do you have a forward assist? Are you hitting it with your palm to get the bullet seated? If you do, can you extract the unfired round? If yes, then are there any gouges on the case or bullet itself? With the upper and lower seperated, how much force would it take to close the bolt and bolt carrier fully on a round?
As far as cleaning goes, shit can accumulate under the extractor and cause problems. If you can, remove the extractor and look for brass/steel shavings and other buildup that may be preventing the extractor from popping over the new case rim.
Clean your chamber good. Steel casings leave powder residual in the chamber due to it not expanding. This is definitely not a short stroke since it is pushing a new round in.
Thought of the buffer spring also comes into mind. It may not be strong enough to overcome the friction.
Quick question, did you have weight on the magazine as you were shooting?
I’d like to stick to PMAGs just to keep things consistent, but if all else fails, I’ll certainly explore this suggestion. Thank you.
I will look into all of these things when I get home from work today.
As with the above, I’ll look into this when I get home.
90% of the ammo I have put through this has been brass casing, so I don’t THINK that’s the issue. Other people have mentioned the spring as well, but I have a hard time believing that a brand new spring would be causing this. Although, defects do occur. I’ll measure the spring and check tolerances and contact Palmetto State if that turns out to be incorrect.
As to the “weight on the mags”… I spent an hour or so on Saturday shooting from the standing position with the mags un-touched. The only “weight” on the mags were the Ranger Plates on the bottom, but that’s obviously not anything to speak of.
You did say you couldn’t push the round all the way into the chamber with your finger right?
Does the bolt close all the way (lock) without a round in the gun? If so, I’m with those who say clean the chamber. If not, look at the bolt (lugs) , can pin and the gas key and tube for battering.
I would not force it using the fwd assist and if you can’t push a round in by hand, I don’t see how the action sprinig can be the culprit.
Ok. So far, your post seems to be most in line with what I would have assumed was the issue. The bolt closes and operates normally without a round. As I said, there is a large amount of friction preventing the round from going all the way into the chamber. I cleaned it pretty well, though, and it doesn’t seem to help. It’s like the chamber is too small by a microscopic amount (which I doubt, given BCM’s reputation for quality) or the casings are a hair too large. I supposed this is possible, but I have never heard of this happening. Anyone care to shed any light?
Look at the problem from cheapest to replace to most expensive. Right now, you are looking at the buffer spring, possibly the mag catch (doubtyful but worth a look), and then possibly a burr in the chamber. That is what I would look at, and in that order for this type of problem.
From:
TECHNICAL MANUAL
ARMY NO. 9-1005-319-23&P
AIR FORCE TO 11W3-5-5-42
RIFLE: 11 3/4 Inches (29 85 cm)
minimum to 13 1./2 inches (34 29 cm)
maximum
Make sure the chamber is clean with your choice of gun solvent and a good chamber brush. After a good scrubbing see if the round can be placed in by hand. If it’s getting further in or any easier then scrub it again. I’m guessing the ammo type might be dirtying up the gun.
Your symptoms are consistent with debris in the chamber. Even with a weak action spring, a clean chamber would allow the round to be seated with a light tap from the forward assist.
The other place to check for debris is the inside the barrel extension.
Another possibility is that some dimension with this lot of ammo is too large. Or the chamber is on the tight side and the previous lot was small. But it’s more likely there is debris clogging up the works someplace
