AR failure to feed, and other issues

[OCB]

So before I get into this, I've already done research on this topic at AR-15.com as well as some other places, and haven't really found a suitable answer. My rifle is short stroking - that much I know. It's a rifle that I put together: a complete bushmaster 16" upper, Stag lower and LPK. I have less than 500 rounds through it, and this problem has only recently started showing up. I don't see any gas tube mushrooming, the gas key is secure, and I haven't messed with any aspect of the gas system - it is as it was from the factory.
The first time this happened was a couple of weeks ago, and given that the rifle is damn near new, I assumed I didn't have enough lubrication in it, so I got some Milspec and lubed it up prior to my trip to HSC.
Today I tried the following: I have 8 different mags, 4 pre-ban 30 rounders that I bought still in the plastic (so I assume they're new, but hey, you never know), and 4 new 10 rounders that I bought from Midway. I had four different types of ammo, two varieties of .223 and two of 5.56. I loaded one round of a type of ammo into each of the eight mags and fired them - the bolt locked back every time. I repeated this test for each ammo type and all the bolt locked back after the round was fired every time. The one caveat to this observation is this: later in the morning I started noticing that even though the bolt was locking back after the last round fired, when I loaded a new mag in and hit the bolt catch button, the first round wasn't stripped from the mag. Looking at it more carefully, I noticed that it wasn't the end of the bolt that was being held back by the bolt catch, but rather, the end of the BCG. If I pulled back on the charging handle and released it, the round was stripped from the mag and loaded properly. The annoying part is the problem isn't repeatable 100% of the time. at times, I can go through 20 rounds without having a problem, then it'll short stroke, ejecting the spent brass but not loading the next round.
So at this point, I'm fairly annoyed. I'm pretty confident the gas system is in good working order, given the low round count and the fact that it was a factory build (not to say it couldn't have been screwed up by the factory). The bolt and BCG had plenty of lubrication, and the fact that I haven't seen this problem until now leads me to think it's not the buffer spring either. However, I'm still new to AR ownership, so I could be completely wrong.
Suggestions would be appreciated.

 

[vellnueve]

Are you sure you have the right size buffer and spring?

 

[BossHarley]

You will get lots of suggestions, but I am going to throw something out there. I had a very similar problem with my Bushmaster DCM. I was using it for service rifle shooting, and was running into the exact same issue (the catch not catching in the proper place). I tried lots of different things - top quality ammo seemed to solve the issue, but not really. Interesting enough - it turned out to be ---- the buffer tube was a little loose and backing out. Eventually it backed out enough that the buffer stop came free and jammed the entire sytsem up. Unfortunately the buffer threads got real messed up, and I had to replace the tube - works fine now. This was a total factory gun with less than 500 rounds as well. Just take a quick look to be sure it is nice and snug - screwed all the way in.

 

[economist]

Gas is the most likely reason (gas key loose, bad alignment of gas block, etc). But, you have checked many of the gas-related reasons already. Here's a less likely explanation: check the chamber. Does a round drop in easily or is it a little tight? I had a bushmaster upper that did what you describe and the reason as far as I could tell was that the chamber was very tight. And, my guess anyway, was that after 20 rounds or so of quick fire things would heat up, brass would start to stick a little, and the upper would appear to short stroke.

 

[OCB]

You will get lots of suggestions, but I am going to throw something out there. I had a very similar problem with my Bushmaster DCM. I was using it for service rifle shooting, and was running into the exact same issue (the catch not catching in the proper place). I tried lots of different things - top quality ammo seemed to solve the issue, but not really. Interesting enough - it turned out to be ---- the buffer tube was a little loose and backing out. Eventually it backed out enough that the buffer stop came free and jammed the entire sytsem up. Unfortunately the buffer threads got real messed up, and I had to replace the tube - works fine now. This was a total factory gun with less than 500 rounds as well. Just take a quick look to be sure it is nice and snug - screwed all the way in.

Interesting, I didn't think to check that... I'll take a look.

Gas is the most likely reason (gas key loose, bad alignment of gas block, etc). But, you have checked many of the gas-related reasons already. Here's a less likely explanation: check the chamber. Does a round drop in easily or is it a little tight? I had a bushmaster upper that did what you describe and the reason as far as I could tell was that the chamber was very tight. And, my guess anyway, was that after 20 rounds or so of quick fire things would heat up, brass would start to stick a little, and the upper would appear to short stroke.

I'll take a look, but I'm inexperienced enough with ARs that I'm not sure what would be considered "too tight". I'm assuming the round should fit someone snugly in the chamber, otherwise you'd have gas escaping once it had been fired.

Are you sure you have the right size buffer and spring?

No, I'm not. The buffer and spring came with my stock (from RRA). I was going to check on that after reading about people getting the wrong spring on AR15.com. However, any idea as to why this problem showed up now as opposed to when I first started shooting the rifle? I guess if the rifle produced higher force on the buffer and spring when it was new, but as I'm breaking it in, the forces reduce a little (as it gets into it's 'normal' operating range), it could cause the problem. maybe. I'm basically making stuff up now...

Thanks for the suggestions, guys.

This morning I was trying to think of all the changes I've made to the rifle and what, if any, impact those changes could have - the only thing I came up with was having the compensator replaced with a muzzle break. The problem started showing up after I had this done, so maybe something during the machining process loosened up the gas tube or something. I plan on checking that out today.

 

[abomb60]

Short-stroking is almost always caused by a gas system problem. Check the following:

1. Gas key is tight and properly staked.
2. Gas ring gaps aren't aligned.

If those are ok then you might have a problem with the gas tube itself. I had a low-round count bushmaster that quickly developed a short-stroking problem. Turned out the back of the gas tube was or got flattened somehow causing a gas leak. If this happens replace the gas tube. They're like $12.00 and can be replaced by removing a single pin and don't require any gunsmith skills.

If you're still having problems after that send it to a gunsmith.

 

[OCB]

Are you sure you have the right size buffer and spring?

So I emailed Rock River to check the weight of the buffer that comes with the stock that I bought, and they emailed back that it weighs 5.2oz. Checking on a few other sites, it appears that this is on par with the weights of H3 and rifle buffers. I understand that using a heavier buffer would reduce the felt recoil, but could this be what's causing my rifle to short stroke???
I checked the gas tube for any deformations and didn't see anything. While I don't believe that my gas key is staked properly, it doesn't budge even the slightest bit.

 

[Skald]

who makes the BCG you got? If its a RRA based BCG odds are its not properly staked and they used locktite to secure it (I bought 2 RRA's but had the shop I got them at to properly stake them).

If you can post a pic of the top of your BCG we can tell quickly if thats the problem or not. And to get the BCG properly staked can be as low as free to $10 or something.

 

[OCB]

I bought a complete Bushmaster upper, so I assume the bolt and BCG are bushmaster parts, unless they bought from another manufacturer. While I'm guessing the gas key isn't staked properly, I don't believe that it is the cause of the problem - the key is very secure on the BCG (for now at least).

 

[Skald]

I bought a complete Bushmaster upper, so I assume the bolt and BCG are bushmaster parts, unless they bought from another manufacturer. While I'm guessing the gas key isn't staked properly, I don't believe that it is the cause of the problem - the key is very secure on the BCG (for now at least).

Again, there is a reason people are saying to get the BCG properly staked. If its not properly staked/secure it becomes a point of failure in the rifle, and can lead to FTF, FTE etc. Staking is pretty cheap and takes guess work in the future for troubleshooting proplem

 

[815187]

Sounds like you might have more than one problem. Bring the rifle to me, and I will fix it for free. I will also show you how to properly adress function issues. It should only take a short peroid. Bring a friend or ten, and we will go to the range after.

 

[OCB]

Sounds like you might have more than one problem. Bring the rifle to me, and I will fix it for free. I will also show you how to properly adress function issues. It should only take a short peroid. Bring a friend or ten, and we will go to the range after.

Thanks for the offer. PM sent...

 

[op4guy]

Just curious if you ever got this fixed? I too would be willing to put money on the gas system and not the buffer. My first suspect would be the gas key as others have mentioned. If the two hex bolts are even minimally loose it can cause this issue. I have also seen some bad pin jobs on the front of the gas tube in the block that has split the tube but it was really hard to pick up on.

As far as gas rings go, alignment has never made a difference in any gun that i have used. I have even gone as far aligning the rings and it never caused a problem. I have even pulled a ring so the bolt was running on two at it still shot without issues.

Just a little input from me.

 

[OCB]

I was finally able to take my rifle to the range on Friday. Since I last posted, I had ordered an H buffer from Spike's. When I went to replace the buffer, I found that RRA uses a rifle length buffer tube, spring, and buffer in the fixed-length "carbine' stock that I bought. No where on the website does it mention that it is a rifle length buffer setup. So, I bought a collapsible stock, pinned it, and installed it with the H buffer onto my rifle.
I observed during my previous testing that when the last round was fired, the times when the BCG was locking back after the last round (which wasn't every time), the bolt catch was engaging the end of the bolt carrier and not the bolt.
On Friday, I tested with eight different mags total (4 30round pre-ban and 4 10 round new mags) and four different types of ammo, 2 varieties of .223 and 2 kinds of 5.56. I loaded one round of the same kind of ammo into each of the 8 magazines and checked that the bolt locked back to the proper location after the round was fired. This was repeated for the four types of ammo. Next, I loaded three rounds of one type of ammo into each magazine, fired the rounds and checked that the bolt locked back properly - again, this was repeated for each type of ammo. During the entire test, I only had two instances of the bolt not locking back the entire way - both were with the same type of ammo in different mags (during my previous test, I had a failure to feed approximately 80% of the time). At the end of the test, I loaded a 10 round mag, and fired all 10 rounds reasonably quickly, with no issues. I then loaded a 30 round mag and emptied it, also with no issues.
Hopefully this means that I've resolved the issue. I still plan on having the gas key staked properly, but as of now, the thing doesn't budge.

Thanks for all the input.

The ammo I used was:
Remington UMC .223
Federal bulk pack .223
WWB 5.56
LC 5.56