I have 10.5" SBR  that a buddy of mine built for me.  It is having some odd issues that I will describe below, including photos.  

Symptoms:  Rifle will fire a few rounds (maybe) and then usually fails in a "bolt over round" failure like shown.  But sometimes (less often) the bolt will be almost closed, but stuck.  It takes me a considerable amount of force to pull the charging handle back and eject the bad round (I don't try to force it in with the forward assist), which is noticeably bent with deep grooves from the bolt on the side (see photos).  Once I had the bullet separate from the case and the bullet was lodged in the chamber.  The shooting range employee used a rod to remove it.  The star chamber has a bit of brass in it, as you can see in the photo below.  Malfunctions occur both with and without suppressor, but *may* be more frequent with suppressor attached.  I thought I had taken a photo of one of the FTF rounds, but I didn't.  You can sort of see it in the photos, but just picture two deep groves from the bolt, and imagine the entire round being noticeably bent.

Gun configuration:
- Aero precision upper and lower, Ballistic Advantage 5.56 10.5" barrel, low-profile non-adjustable gas block, carbine gas system.
- Gun seems significantly over-gassed.  Ejection pattern is around 1:00 with suppressor.  Without suppressor, it's about 2:30.  With heavier springs/buffer. I could get it to 3:00.
- The issues seem to happen more with a fully-loaded (but 100% certainly not over-loaded) or even mostly-loaded magazine.  But this could just be a coincidence, I wish I had tested this a bit more last range trip.  
- Gun has never had enough rounds through it to prove reliable.  I think a few years back when I first got it, I fired it a limited amount of times.  All I remember from back then were some extraction issues as noted below.

What I've tried (all of which have failed):

- Swap out to a new BCG (Aero Precision to BCM).  Also the bolts pass the gas ring test, standing it up on end
- Different brands of ammo that I believe are considered "good".  I.e. Federal LC 55gn, Winchester LC, Armscore 55gn
- I've run it between somewhat dry all the way to excessive lubrication of bolt, BCG, even charging handle
- Swapped upper/lower with a known reliable 11.5" SBR.  The issues followed the upper.
- Took it to a gunsmith, they said they think it might be out of spec feed ramps and recommended swapping barrels and/or having BA check it out.
- Changed buffers different combinations (carbine, Spikes ST-T2, H2)
- Tried different buffer springs (equivalent of Sprinco blue and orange).  With the heaviest spring, it was very noticeable how hard the bolt slammed shut.  Went back to standard carbine spring b/c springs didn't fix the problem
- Tried many different magazines that run fine in other guns, including both Pmags and aluminum Okay magazines.
- Nothing is touching the magazine while gun is being fired (i.e. I'm not using it to prop the gun up or as a forward grip).


This is what it looks like after most failures.  Round will often be not only dented, but bent, so I discard it.

Another angle

Feed ramps 1

Feed ramps 2

Brass shavings


Other information that may be irrelevant:

- When I swapped upper/lower receivers with a known "good" rifle, I couldn't get the front (pivot) pin all the way in.  So I grabbed yet another complete lower and the pivot pin went in, but I had trouble pulling it back out. So seems pivot pin hole is tight on this upper (on the problematic gun)
- A long time ago I remember this gun having extraction problems of some kind.  Unfortunately I cannot remember exactly what the problem was, but when I got a new BCG that had an o-ring installed, the problems went away.  
- Gun seems to reliably lock back on empty magazine.  However I think ONE time it may not have.


So I am wondering what is going on here.  I've tried to "eyeball" the feed ramps and lightly run a punch to feel out any kind of "shelf" but I can't really see anything wrong.  It seems that at some point the bolt is digging into the side of the round, but then it manages to get in back of the round and tries to chamber it, but then fails.  My plan (for now) is to completely replace the barrel, gas tube and gas block.  I'll see if BA will be willing to take a look at the barrel and see if it's out of spec.  Should I also consider trying a new upper receiver?

Thanks.

I'd change out the upper receiver first.

Try it and see if the issue persists.

If not, scrap the receiver, problem solved.

If it doesn't fix the issue, get a new barrel.

Just to summarize,

This happens both suppressed and unsuppressed?  Also what can is it?

The only items you havent changed out are the barrel and receiver?

If possible can you clean the feed ramps and chamber ? its hard to tell in the picture but the middle lug looks chewed up but its hard to tell.  When i compare it to a new barrel i have it looks different.  

I see that type of malfunction frequently in 10.5 guns with suppressors attached.  When we remove the suppressor the malfunction goes away.  Its a gassing issue for our guns but its hard to tell if that is what is causing it for you.  WIth ours its carrier speed.
Another thing to keep in mind is that BA 10.5 barrels could have a hogged out gas port hole which will exacerbate the issue your having.

The bolt over base malfunction is normally just enough gas to cycle and eject the spent round, but not cycle far enough to the rear to get the bolt behind the magazine to strip a new round.
I would venture to say your gas port is partially covered/obstructed. Or gas port not big enough.

Maybe I am not reading correctly but the bolt is cycling back far enough to grab the rear of the case head as in the picture and denting the case from ramming it forward? Or the picture is after you found a way to get the bolt off the dented part and behind the casehead? That dented part looks like the bolt was scraping it hard.

My initial guess just looking at pics was a magazine failure releasing the round upward too soon while holding the base in too long.

This is where iPhone slow mo comes in the rescue sometimes.  If it’s not a dangerous malfunction and it’s repeatable I like to film it.

https://www.ar15.com/forums/AR-15/Troubleshooting/66/

You're probably about to get a lot of A5 recommendations. I won't agree or disagree with it.  Seems you really need to slow down the cycle.  Heavy springs assist in speeding up the BCG return to battery.  The opposite of what you need.

Or the opposite... an excessive amount of energy and a poor buffer, so that the BCG/buffer stack are rebounding from the back of the buffer tube before the magazine can push the ammo stack up for proper feeding. Which can be exacerbated if there is insufficient bolt travel... if you pull the bolt back all the way, you should have about 1/4 inch of space or a little more between the bolt face and the bolt catch as a rough gauge. Opposite extremes can create similar looking malfunctions.  Ejection pattern is not necessarily a reliable gauge, but something under-powered enough to not be able to cycle is not going to eject at 1 o'clock.

I'd be curious to know what size gas port it has if pulling the gas block wouldn't be too hard, but I think excessive energy is a good possibility.

If it would be possible to test it on a rifle lower that might also be a data point.

ETA: finally got pics to load. Ignore the above. That's not a "bolt over base" malfunction, the round is in front of the bolt, just not feeding into the chamber. Low energy as 50cal described could be part of the problem, along with magazine, recoil spring, drag from the feedramps, or magwell out of spec (although some of those you've probably ruled out).

Your rifle is filthy, clean it.

It looks like the M4 barrel extension is extended past the M4 ramp on your upper receiver, causing a ridge that would interfere with feeding. Did you "true" the receiver face and remove a lot of material before installing the barrel? Shims are available that are designed to help time barrel installation, you may be able to use them (cheap fix) to get a better match between the extension and the upper.

If you can feel a ridge and a stopping point on the feed ramps where it meets the barrel extension, there is no way you are going to have a reliable rifle. Bullet points will get caught trying to feed more time than not. You can remove the barrel and polish the feed ramps, it's an easy option, especially if you own a Dremel.

Short barrels and carbine gas systems are the least reliable setups available. I bought two brand new DD Mk 18's and had to play with buffers to get one to run reliably.

That may be true, but the photos depict rounds already past that point, so that's not really the problem.

Looks like excessive carrier velocity to me but it’s a weird situation with not getting better with heavier buffer and removing suppressor.

Try that barrel on a known good running AR if you really feel like tinkering..

Thanks for the replies so far.  I'll try to answer a few questions here:

- Malfunctions happen both suppressed and unsuppressed
- Suppressor is a Yankee Hill Phantom Stainless Steel 5.56
- I've changed out magazines, even used a totally different lower.  In the upper, I've tried different BCG
- It does look to me in the picture like the middle lug has its left corner chipped off. I wonder if that could cause these types of problems.  I'll see if I can get a closer look, taking barrel off the upper if needed.
- I'll ask my friend if he did anything to smooth out the feed ramps.  I suspect he just put the barrel in the upper and that's it.
- I like the idea of trying to take a video and then slowing it down.  I'll try that next week at the range.

That'd be my first guess too. Gas block hole not aligned with gas port is 1st thing I'd check. On some barrels it's easy to push the block up to a shoulder and actually go too far.

After that I'd work back through the gas tube, alignment with the gas key on your receiver, check gas key for tightness and seal, check gas rings, and verify you have correct buffer spring and buffer. All easy to check in just a few minutes time.

I had a 14.5 bushmaster barrel years ago that did this. Swapped to RRA 9mm buffer and problem went away.  Had too much carrier velocity and the mag springs couldn't keep up.

That right feed ramp looks a little rougher to me, but might just be dirt or the camera. Either way, one other easy thing to do while testing is count your rounds and see if you're getting the failure consistently on one feed ramp and not the other.

I'd still check the whole gas system first,personally. Don't even need to replace anything yet, just make sure everything is aligned correctly.

Added some photos for gas block alignment:

Gas block hole

Gas block upside-down

I don't know how to tell if the gas block hole is large enough for this gun, but it appeared to be aligned properly based on these photos.  I took a photo of the gas block upside-down because I read somewhere that the screw holes should be perfectly aligned with the gas block hole.

I’m still confused if the first photo is the natural state of the jam or after you racked the bolt off the top of the round?   The bolt lugs are behind the round not on top of it.  So is the bolt slamming the round at the rear so hard it kinks the case and creates the dent, or is the bolt battering the case midway up where the dent is, and you pulled the bolt back so let us see the dent?

Measure the port while you have the block off.

This is what it looks like without touching anything, or even dropping the magazine.  It doesn't make any sense to me either, because it looks like the bolt was able to get on top of the round to dig grooves into it, but somehow ends up behind it.




I'll see if I can.  From what I've read on here I might be able to use a drill bit to do it?  I don't have calipers or any special tools like that. ETA:  a 5/64" drill bit fits in (the smooth side).  I guess that comes out to around 0.078".

That's a large gas port.  Chances are that it actually .080ish.  It should be .070-.072.  Get an adjustable GB or a BRT tube.  Like I said before, you're overgassed.

I wouldnt generally think 0.078" is severely over gassed on a 10.5" carbine. However, based on the pics above and the feeling that the suppressor actually increases failure rate I'd lean more towards overgassed now too.

Don't remember from OP but if you have access to an H2/3 heavier buffer it'd be worth shooting a couple times to see if that helps. Ultimately though if it's overgassed you'll want to fix at the source via either a BRT tube or adj block mentioned above, or even a cheap/easy fix like an adj gas key on your carrier.

Since you've had the gas block off I'd do one of the first two options, personally.

There is that whole "operating cycle" thing, where the bolt comes back, ammo gets pushed up, bolt goes forward. There are some edges on the bolt and bolt carrier that can leave marks on the cartridge cases, particularly if they're left sharp after machining.

As others have noted, I have to believe your upper is the problem.

The pivot pin being obstinate, to me, says the upper is out of spec.

Hope this helps as I'm sure it's frustrating. I know I hate to spend precious range time having to work on an uncooperative gun.

Agreed... Alignment is everything to me... Index pin fitted perfectly into the upper receiver's index-pin slot then gas-tube aligned correctly from FSB (spaced/centered correctly over Gas-Port), then into the receiver's GB hole no twist/bind... Lastly the BCG's carrier key true at 12:00 aligned to the gas tube with no binding. Check by going from flat to a 45* turn on upper completely out of battery to see how easily the BCG slides into position and if it the bolt lugs lock into place fully and easily. Then an H2 or H3 Buffer paired with a Springco or JP Enhanced buffer spring should set you strait as long as the barrel and feed ramps are timed true, no burs, no gaps to jump, etc...

ETA: After looking at those pics magnified more correctly... Either the Upper is out of spec, it's index-slot isn't timed right... Or the barrel-extension's itself isn't timed right and/or the index pin isn't as well. You got some lug mismatching between the upper and barrel extension which is ever going to run right until it's all timed correctly with shims or replacement of the offending component...

Any updates?

Thanks for asking.  I do have some things to report.  I believe what I have done might be a little controversial, but I will go ahead and share it anyway.  Because I had a gunsmith tell me that he believed something may be wrong with the barrel itself and/or feed ramps, I went ahead and ordered a brand new 11.5 in barrel from Aero Precision, a new low profile gas block and a new gas tube.  I kind of wanted an 11.5 in barrel anyway, and looking very closely at the existing barrel I can tell that something a bit odd was going on with the barrel lugs compared to other barrels that I have (and you can see it in the picture as others have commented).  I'm working with the manufacturer to see if they will take a look at it.  Also, I had also never assembled an upper before so I wanted to do that just for the experience, and of course I followed all the advice that people on this website give about the shell anti-seize grease, the exact torque values to use, how to line up the gas block, etc.  

Even with the new barrel the problems persisted.  So I swapped out the bolt carrier group for a new BCM one. It no longer malfunctioned.  I then inserted a toolcraft bolt carrier group, and it still functioned flawlessly.  So I was curious if it was just the bolt itself or the entire BCG.  So I swapped the bolts between the two carriers and the malfunctions immediately occurred wherever the bolt went.  I don't know exactly what the problem is with that bolt.  I Don't know if this could have anything to do with it, but it seems either the ejector spring or the extractor spring is a bit weaker than all the other bolts that I have.  Meaning if I try to seat a round into the bolt just by hand (flat against the bolt face with rim under the extractor) I do not have to push very hard, It goes in quite easily.  On all my other bolts it takes quite a bit of force to get it in there.  So maybe all of you can tell me what that means.  Either way it now performs flawlessly through about 200+ rounds out of many different magazines, suppressor, no suppressor, clean, dirty, lubed, etc.

In my original post I had reported that I already had tried swapping out to a new bolt carrier group.  It's possible that I was mistaken and I somehow screwed up the testing, or maybe it was some kind of weird combination of the bolt plus the barrel.  Or maybe the over-gassed original barrel was enough to cause the problem on the new bulk carrier group. I don't know. But either way it runs very well now.

Try completely rebuilding your problematic bolt with new parts from a known vendor. Gas rings also.
Completely strip your bolt and rebuild it.

This should be a lesson to all those who religiously tell new people to “just build it yourself, you can save money.”  Maybe.  Or maybe shit goes sideways.

Glad it’s working.

Probably gas rings if it was working fine before.

While I didn't have any failure, yesterday a gas ring failed on me. The telling sign is that if you put the bcg into the upper and tilt it, it would just slide right out even in battery.

It was very hard to notice at first but one of the gas rings snapped. If I didn't look carefully it would appear all 3 were there. It passed the bolt standing test.  I wish I would have taken a picture but I took all of the gas rings out and one had a break where about 80% of it was still intact.

I changed out the gas rings and bam, back to normal.

As a last ditch effort, you can simply try swapping the rings out to see if it helps. I just read the other guy's post above and I think a rebuild is a good idea. New gas rings, extractor, extractor spring, ejector and the ejector spring. You can buy them generally for about 20 bucks or so.