Just my luck AR15 problems...

[FullEffect1911]

Alright, I have posted a lot of my ar15's problems in various sections of this forum. However at those times I thought it was an ammo problem.

The rifle is a 20" bushmaster ar15 A2 with less then 1000 rounds through it. The rifle is very clean.

Here is the situation.

Using various types of factory and handloaded ammunition, the rifle will short stroke and fail to lock after the last round.

Occasionally the bolt will just skip over the next round in the magazine and force me to chamber the next round manually.

Sometimes the bolt will start to pick up the next round, however since the bolt didn't travel all the way back it will jam the round and dent the case. This of course forces me to clear the jam. I believe this would be a 3 point jam.

I do have some powder leaking past the front sight base, however I have come to believe some leakage is normal.

The overwhelming majority of the cases show some shinny scrape signs in the extractor groove. These marks lead me to believe that the rifle is unlocking and trying to extract the case that is still under pressure and pressed up against the chamber walls. This would bleed off momentum of the bolt/bolt carrier and cause short stroking. As I have read this would be an over pressurization of the gas system.

The tube is not bent. The bolt key is tight and still staked. The tube's mushroom looks fine. Malfunctions happen with both my bushmaster and colt mag. Gun is clean, gas system was cleaned with a pipe cleaner, bolt carrier was lubed up nicely and the buffer spring tube has a light coating of oil. The bolt it pretty well worn together with the upper so it is pretty smooth. The bolt will lock on an empty magazine if I do it manually. I am 99% certain it is not ammo related (my most recent ammo try was black hills 55grain).

I have not taken pieces off the gun yet, as I have never fully disassembled an AR, however I would really love to solve this problem and I need help. If anyone has any ideas on where to go from here I would be very appreciative.

 

[Onmilo]

Take a look at your bolt carrier.
If the underside is machined away enough that you can see the bottom of the firing pin this may be part of the problem.

Try repolacing your bolt carrier with one that features a support web in this area.

Bushmaster Bolt carriers can be a problem in my opinion, the machining on them is kind of half-assed and the carrier key assembly isn't always lined up correctly either..

Go here:
http://www.brownells.com
and order a High Standard bolt carrier assembly, these feature support in the firing pin area and the machining is much cleaner.
You don't need another bolt unless you want one for a spare, get a field headspace guage if you buy one just to be sure headspace is OK, but you just need the bolt carrier assembly.HTH

 

[ogree]

When you get a chance, pop out the retaining pins of the front sight base and remove the front sight assembly.
Check to see if there is any visible carbon buildup on the exterior of the barrel around the gas port, if you do find some carbon around the port in the shape of a half moon by some chance, your front sight is mis-aligned and is impeeding the gas and not allowing the full measure to be directed into the tube.
Hence.......short stroke.
This is not a common problem but I have seen it from time to time and baffles alot of people when no other signs can be detected for a malfunction like you have stated.
In this rare case, the only options are to weld up the cross pin slots & re-machine them, replace the barrel or replace the front sight base.

This is rare though and odds are it is not causing your problem, but doesn't hurt to check.

 

[asknight]

From what you're saying, I bet it's short-stroking on all rounds, its just that the bolt happens to be changing directions between the rear wall of the magazine and bolt stop, and really short stroking when it fails to completely pick up a new round.

The gas leakage is not normal. The only place the gas is supposed to vent on the AR is the muzzle and the gas key on the carrier. Either your front sight housing (gas block) is out of spec, or you've got some serious gas port erosion. I'm also willing to bet that this problem would rear it's head earlier on a 20" rifle before it would on a 16" CAR due to the tremendous increase in the total amount of gas available in the CAR.

If that's a factory assembled Bushmaster, I'm certain they would take care of it for you.

Onmilo, isn't the problem that you describe the one that usually results in premature hammer wear, with the carrier not resetting the hammer each time?

 

[FullEffect1911]

my guess is that it's short stroking on every round also. I think this one deserves a trip to bushmaster.

 

[Onmilo]

I have seen where the relieved bolt carriers carve two nice grooves in the hammer but I wouldn't call it premature wear, more like unecessary wear.
These relieved bolt carriers cause problems with quite a few of the two stage triggers too.

There is another problem that you may be experiencing I thought about last night since you mentioned some gas leaking around the front sight/gas block.

Your gas tube may not have been drilled correctly and the through pin hole may not be lined up exactly perpendicular with the gas port hole in the tube.
In other words, to get the through pin hole lined up with the gas block may be causing the gas port hole to be off center to the barrel gas port.
Not enough gas getting out of the barrel and down the tube= short cycling.
The gas tube may not be correctly lined up through the upper receiver and it is binding on the carrier gas key too.

Sending the rifle back to Bushy sounds like a good idea.

 

[Alice Cooper]

signs of a bit of gas leakage around the front sight is normal.there is no need for a airtight fit there,as there is plenty enough pressure to operate the bolt. I assume the rifle didn't get to the 1000 rd. count with this problem, so the gasport is not missaligned.go to ar15.com and ask Dano,he knows more about ar function/malfunction than anyone i've ever seen...

 

[usp_fan]

Is the carrier gas key tight?

 

[asknight]

there is no need for a airtight fit there

 

[FullEffect1911]

The Rifle is on it's way to bushmaster. They agreed without much fuss to take care of the problem. However they did kinda *yell* at me about using wolf. They specifically asked if I used moly coated wolf (with the green tips), I did not however. The customer service person was quite nice to me and didn't give me a hard time. Though if I hadn't used blackhills ammo to really test the gun they would have told me to use better stuff I am sure. Oh and they paid shipping.

Anyway i'm hoping things will be resolved shortly and i'll report back.


PS gas key was quite tight, and staked.

 

[Alice Cooper]

ok, "gastight", meaning around the hole where the gastube enters, not the whole front sight.i',m betting a real GOOD chamber cleaning would fix it...