Bolt Damage.

[alx01]

An in spec rifle cannot generate contact like that on the front of the bolt lugs. Cannot.

The geometries do not exist to permit this to happen.

I'm not an armorer and can't say what, in theory vs practice, can and cannot happen on a non-standard bolt. But OP did his due diligence, more than most of the people, to check and gauge things. He sounds like a very detail-oriented guy who knows what he's doing with proper gauging.

All I'm saying is: I suspect if there was a significant mechanical/tolerance issue it would have manifested itself more rapidly and had other side affects besides only bolt peening in specific points. Not saying I'm right in this particular case.

If you think it is common to have issues with the items you've mentioned (parts geometry and tolerances) and not notice any other effects, then it's definitely a possibility.

[BufordTJustice]

I'm not an armorer and can't say what, in theory vs practice, can and cannot happen on a non-standard bolt. But OP did his due diligence, more than most of the people, to check and gauge things. He sounds like a very detail-oriented guy who knows what he's doing with proper gauging.

All I'm saying is: I suspect if there was a significant mechanical/tolerance issue it would have manifested itself more rapidly and had other side affects besides only bolt peening in specific points. Not saying I'm right in this particular case.

If you think it is common to have issues with the items you've mentioned (parts geometry and tolerances) and not notice any other effects, then it's definitely a possibility.

A discussion of statistical probabilities doesn’t really speak to this specific issue.

I’ve stated that you cannot achieve the physical relationship between the bolt lugs and barrel extension if all parts are in spec. I don’t understand the impetus to rule out all variables other than the bolt without picking up a set of gauges and calipers. I have said to scrutinize everything.

I can’t speak to a person’s threshold to “notice” something.


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[ScottsBad]

I've got a couple of those with enhanced carriers. I've not experienced that kind of wear, but mine only have a maybe 200 rounds each. And I'm not running a shorty with a can.

Doesn't look right to me.

Good luck hope it works out OK.

[1168]

I’ve got 6,000+ rounds on one at home in a regular carrier. I’ll try to remember to take a look in a day or two.

[Core781]

Chamber gauge all three gauges. Looks like something is out of spec. Probably should have gauged it upon initial assembly and checked for resistance on insertion and extraction with GO gauge. Good luck! Get a new bolt and gauge it or RMA the upper to a smith/armorer or manufacture.

[LMT/556]

If you index pin egged or sheared during assembly the barrel extension might be misaligned so the bolt strikes going both in and out.
https://www.youtube.com/watch?v=iJ2KhjAu3ME&t=496s this example is a PSA built rifle with a slightly clocked barrel.

LMT Enhanced bolt and carrier, bolt face diameter was small, as in tight.
https://www.youtube.com/watch?v=I_9913p57Us
https://www.youtube.com/watch?v=RUxiXZcfsFM

[1168]

I’ve got 6,000+ rounds on one at home in a regular carrier. I’ll try to remember to take a look in a day or two.

Cleaned and inspected said bolt. Overall, mine looks waaaaay better than the OP’s. There is some peening in spots that is hard to see, but I can feel it with a fingernail. Hmmm.

Mine’s been used with a BRT 16” barrel and some fairly hot ammo, along with the usual diet of whatever brass cased stuff is available. Mostly 5.56, though. I imagine it’ll still go for another barrel worth, but I guess we’ll see.

[sig1473]

Any update on this?

[constructor]

Size of the contact patch matters LOL. LxWxYield strength. With all of the chamfered corners the contact patch has been reduced so the thrust is concentrated and those chamfered edges are the largest I have seen on any bolt. BTW the length of the lugs should be .277, if shorter they may have compressed and enlarged your headspace.

[Rogue556]

Update:

After one of BufordTJustice's posts, I wanted to comb over everything and see if there could be anything else causing the issue.

So I finally got time to really dig into this issue and found something interesting.

I grabbed every other rifle I own and compared this one in question to the rest, and I noticed that this particular setup seems to drag ever so slightly going into battery vs my others. After thinking about it, I've actually assembled this upper three times, not counting the most recent. I know what processes I went through the first time, and everything was good to go. During my second assembly was when I encountered issues with the out of spec LMT receiver, and I ended up returning it and using my original receiver again. The third assembly, using the original receiver, I am not entirely sure that I went through everything with a fine tooth comb as I did the two previous times. Frustration may have got to me a bit, who knows.

So after this realization, I looked at it again and noticed that as you ride the BCG into battery, the tension would start just about the time the gas key would slip over the gas tube in the upper receiver. I pulled the BCG out and tried it in my other uppers, and there was no hang up what so ever.

So.. I call LMT and finally get ahold of the individual I'd been working with. I told him what was going on, and explained that the gas key is apparently hitting the gas tube, causing binding. I already had an RMA on its way at this time, and I told the individual I'd just discard the RMA and deal with the damaged bolt. He agreed that would be the best course of action, as they could not warranty the bolt if the damage was user induced. That was the last conversation I had with LMT regarding the issue.

Jump forward a few days, and I decide I'm going to go through the upper again and get it reassembled correctly. I pulled the handguard to check the gas block and tube, and everything appeared good externally. I pulled everything apart and rechecked heads pace. Still good. I reapplied Aeroshell 33ms to the barrel extension and receiver threads, and torqued the barrel nut down to the recommended 40 ft lbs. At this point I start checking the gas tube alignment using a bolt carrier with the bolt removed.

This is where things got wonky..

The carrier had no binding what so ever in the upper. Surprised, I grabbed another carrier, removed the bolt, and tried again. No resistance again.

I was surprised as I figured I'd have to do a little work to get the tube realigned perfect, if it was indeed that far outnof wack last time.

I decided to install the bolt back into the carrier and try again.. and what do you know, the dragging is back. I grabbed a few other complete BCGs and used them as well. All of them had some kind of resistance, but when used in their own uppers they freely glide in and out of battery.

It's now apparent the gas tube was never the issue. I didn't catch this the first few times I assembled the upper, because I checked the gas tube with a stripped carrier, and assumed all was good since the bolt passed the gauges. The third time I assembled the upper, I didn't check it with carrier only. I assumed it had to be the gas key causing binding.

The lugs on the barrel extension are tight enough that the bolt, even with perfect alignment, is making unnecessary friction going in and out of battery. It does this with all of the bolts I have, but the BCM I've been running since the issue began seems to still be holding up ok. I think what BufordTJustice said regarding tolerance stacking is the most likely culprit. I've heard others mention the enhanced bolt being tighter than standard bolts (not sure if that's true). It's still entirely possible that the extension, the bolt, or both is out of spec.. but since the BCM bolt that is currently in the rifle is not showing the same wear, I'm going to chalk it up to tolerance stacking for now, unless this bolt shows the same wear after I've run it for a while.

How many of you guys have seen this before? I really never stopped to consider the barrel extension lugs being too tight, causing premature bolt wear, but it's now making sense. Is it a common occurrence or out of the ordinary?

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