Bolt Damage.

[Core781]

When you Frankenstein an AR together you should be aware of the entire operating system as you assemble it. Most DIY carbines I see at any training event tends to have significant issues or just malfunction taking it out of the game. You can avoid all of this by taking a reputable armorers course. When you get the knowledge to identify parts that don’t play well together, you can return them before you potentially blow up your AR. I had a shell get crammed in the upper once and do some pretty serious damage due to an overgassed setup. Most courses are reasonable and definitely worth it to potentially save your life if you plan to keep assembling your own ARs. We had a kid at the last event who assembled his own from the cheapest parts I have seen, it would not cycle period. He did not align the gas port correctly when he installed his clamp on FSP which I’m fairly certain he probably didn’t dimple the barrel and managed to distort the bore enough to render the carbine unserviceable. There’s a thousand ways to screw these things up if you do not understand the basic process. When you introduce enhanced parts it can make things ever more perplexing. Due to my lack of a proper mill I am limited to drill press and hand tools. Certain things are best left machined if possible or require skilled and experienced hands. So far the most complex thing I have done is install a FSP with taper pins, welding and coating. For the most part it’s function testing and knowing what to check and troubleshooting until you identify the issue if things don’t fit smoothly. My last problem was an over stacked lower, upper, and BCG and the loaded mags were beyond unreasonable to get them locked in: when I filed the catch assembly to the best of my ability, it turned out to be the combination of the lower, upper, and BCG. I have never seen a build that tight. After enough mag insertions, it resolved: but there is no diagnosis for this in my manual, or my experience until that aha moment. So it required skilled filing, which could have ruined the latch assembly and a wear in period. When I prep a mil spec FCG it is almost an art form to get it right: and in some cases a trigger or hammer will never jive well together: and you have to get a new set. It sounds like something is off with the upper and barrel extension given repeated conditions with different bolts as you said. Never had a Colt extension out of spec but there’s a first time for everything and I have seen Colts with some serious issues slip through QC. If I buy a firearm, I field strip it at the gun store and inspect everything and function check everything. I still manage to miss little issues, but it’s saved me more than once already when purchasing pistols that had issues. A good extension gauge set is around $100 at Pacific Tool if you want to check the barrel extension. Good luck!

[hotrodder636]

Thanks for the update. Glad to see you found the apparent cause and shared it with us, not just keeping it in the dark.

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?

Sent from my SM-N910P using Tapatalk

[BufordTJustice]

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?

Sent from my SM-N910P using Tapatalk

Thanks for the update. Tolerance stacking happens.

Kudos for doing the proper troubleshooting.


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