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FXR
10-12-2011, 9:27 PM
Hi all, I just put together an AR and I'm having some issues. Everything starts out fine, but about 50-100 rounds in (on 2 range trips) it starts jamming. The bolt strips the round out of the mag, but then slips on top of it it and winds up denting the side of the casing about halfway down. It gets bad enough that it was repeatedly happening every round for the last 5 or so rounds before I put it away. Also on an unrelated note, if I do make it to the end of a mag, it seems to only hold the bolt open about 50% of the time, and does not appear to be mag dependent, although I haven't checked this out thoroughly enough.

One possibility is that the carbine gas system isn't happy with the rifle buffer; thoughts?

Components:
-KD lower with unknown LPK; for what it's worth it has a real nice trigger pull and an expensive ambi safety so probably not bottom shelf parts.
-RIFLE length buffer
-Used Colt 16" HBAR A3 upper (Carbine length gas system)
-New Colt M16 BCG
-Mucilek muzzle brake
-30 round P-mags (featureless)

I'm shooting PMC green tip 62gr 2010 manufacture and lubricating with Hoppe's.

joelogic
10-12-2011, 10:05 PM
The gas system and buffer dont have to match.
The buffer and stock need to match.
You didnt list stock type.

wash
10-12-2011, 10:17 PM
Sounds like it might be undergassed or something is dragging once it gets fouled.

Disassemble the BCG to see if there is carbon buildup or rub marks. It could be the bolt lugs and barrel extension too.

I usually just add more break-free CLP to keep my AR running, wet is better. I haven't tried Hoppe's

Make sure your buffer spring is right, that's the only other thing I can think of.

triggs75
10-12-2011, 10:38 PM
have you tried more then one mag?

kemikalembalance
10-13-2011, 1:29 AM
spend the money for at least one or two good quality medal mags. try other peoples stuff. if you can...try a buddies lower. Unlike a wife, you can swap AR parts with a buddy to see if his IS really better. oil the buffer spring with some regular gun oil. and ive heard of cutting one coil off of the spring to increase the bolt speed. but i dunno enough about that yet so ask. And how hot is it getting when it fails? are you gettin to 100 as fast as you can? Or a couple here and there till the box is empty?

FXR
10-13-2011, 2:09 PM
I just cleaned the BCG - there was a fair bit of buildup in there. No unusual wear marks. I'm off to the range in a couple hours so we'll see if that did it.

How do I know if I have the right buffer spring? Am I correct that the carbine buffer has a stronger spring because of the stronger gas pressure? My symptom sort of feels like either not enough gas or too much spring.

FXR
10-13-2011, 2:09 PM
I just cleaned the BCG - there was a fair bit of buildup in there. No unusual wear marks. I'm off to the range in a couple hours so we'll see if that did it.

How do I know if I have the right buffer spring? Am I correct that the carbine buffer has a stronger spring because of the stronger gas pressure? My symptom sort of feels like either not enough gas or too much spring.

21SF
10-13-2011, 2:39 PM
Check your mag fit, and mag catch. Sounds like you mag may be sitting low.

rero360
10-13-2011, 4:38 PM
You should not have a carbine length spring with a rifle length buffer. You mentioned that the rifle is featureless, however is the receiver extension rifle length, carbine length or pistol length?

razorscs
10-13-2011, 5:04 PM
I've had the exact problem that you're having with one of my AR's and can't figure out what's causing it for the life of me. It's gotten to the point where I have decided to just put it away for a while and not shoot it because it's pissing me off so much haha.

wash
10-13-2011, 5:19 PM
The number of coils and free length are different between carbine and rifle springs. Google can tell you the difference, I'm lazy.

For all those asking, he obviously has a rifle buffer tube, a rifle buffer in a carbine, entry or pistol buffer tube would not work at all.

I hope that cleaning your BCG does the trick.

Merc1138
10-13-2011, 5:38 PM
Rifle buffer, rifle spring, rifle tube.

Carbine buffer, carbine spring, carbine tube.

You should have no other combination besides one of the two that I just listed. The gas system doesn't matter. We also have no idea what lube you're using, and how much is actually on the BCG. Your AR should be "wet", if you clean and lube it, and you can describe it as being dry when you're done, lube it again properly.

You mentioned 30 round pmags, have you tried a different mag altogether?

You also mentioned that the upper is used, have you tried it with a lower that is known to be working 100%?

RLTW
10-13-2011, 5:58 PM
Unlike a wife, you can swap AR parts with a buddy to see if his IS really better.

That's Funny...

FXR
10-17-2011, 2:21 PM
Well, cleaning out the BCG seems to have helped - I had one of these jams and one failure to lock back out of 200 rounds. To clarify, I'm using 6 different Pmags and now I think the not locking back is not mag dependent. Also, the whole rifle assembly is Rifle. I'll look into borrowing a lower and comparing mag seating heights.

wash
10-17-2011, 2:40 PM
Well, if there is no other obvious problem, I have to assume it is under-gassed.

One more guess:

If the upper you have was "used" with a drop-in .22lr kit, you could have lead and other junk in your gas port and gas tube. That might have made your bolt carrier dirty and explain the problem getting better but not going away.

If you take the gas block off your barrel and find that your gas tube is full of junk, clean it, the gas block and your gas port to fix the problem.

If you find the gas tube, gas block and gas port clean, and you think it is undergassed, check for gas port to gas block alignment and if that is good you probably want to increase the gas port size. Just remember that you can't go backward after drilling it larger and don't drill in to the rifling on the bottom side of the barrel.

sir9usr
10-17-2011, 2:50 PM
Had the same issue in my Colt M4 - in your case it appeared to get a little better after cleaning the BCG so I'd advise checking the 3 gas rings in the rear of the bolt. The gaps shouldn't be lined up (120 degs apart) or just replace them (they're much cheaper than drilling out the port in the barrel). Some manuals recommend a drop or two of clp on the rings before re-assembly.

Peter.Steele
10-17-2011, 5:13 PM
Rifle buffer, rifle spring, rifle tube.

Carbine buffer, carbine spring, carbine tube.

You should have no other combination besides one of the two that I just listed.



I have a different combination! :D

A5 buffer, rifle spring, A5 tube!

ZX-10R
10-17-2011, 5:41 PM
AK, AK Pistol, AK...That is the combination :p

rero360
10-17-2011, 6:08 PM
I'd advise checking the 3 gas rings in the rear of the bolt. The gaps shouldn't be lined up (120 degs apart)

BS, pure and simple, you do not need to stagger the gas rings, heck you can shoot the rifle just fine with only one gas ring and it will function.

But they are cheap, and easy to swap out. If the rifle is undergassed then I'd put my money on the gas block/ tube as being the failure point, not the gas rings. Most civilians simply do not shoot enough in their whole freaking life time to wear the rings out to the point that they would cause failures.