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Apoptotic
10-11-21, 14:13
I ordered a complete 14.5” carbine gas length AR upper in 5.56 from a large manufacturer with a sterling reputation for reliability/build quality. I mated this upper to a lower from the same company. For a long while I could only shoot this rifle at a range that only allowed slow firing, i.e. greater than one second between trigger pulls. Shooting at this range, the rifle was 100% reliable. Recently I began shooting at a new range that allows rapid firing. Whilst firing rapidly, the rifle would have 1-2 malfunctions per 30 round magazine. Each malfunction ended with the expended brass becoming wedged between the bolt face and the distal, inside edge of the charging handle. I tried many things to fix the issue but without success. The other day, I decided to look at the bolt for issues but all seemed well until I noticed a small stamping that reads “6.8mm” (presumably 6.8SPC). I’m embarrassed that I never noted this before when cleaning the bolt or when I was trouble shooting it previously (sometimes you don’t see a problem that you’d never look for in the first place). I replaced the entire BCG with a new one from the same company and fired 100 rounds through it, encountering one failure to feed. So, in the short term, the issue seems to have been remediated.

I contacted the company and they have sent me an RMA. I’ll be sending the upper in tomorrow.

Obviously, this company should not have sent me a 5.56 upper with a 6.8 bolt in the BCG. Mistakes happen, and I’ve only posted the aforementioned history of the upper to properly set up my question to you.

Given this upper has 600 or so rounds through it with the 6.8mm bolt, should I expect damage has been done to the bore or any other components of the upper? I can’t see anything grossly wrong, but I don’t have a trained eye. I’m asking for your thoughts because I want to be able to have a informed discussion with this company once their armorers have the upper in hand. I want to ensure they have done more than a cursory investigation for damage if it seems there could be long term consequences that may not necessarily manifest at this time. Would any of you, for example, go ahead and change out the barrel just of the basis of the history I provided above? (I’m not fishing for a new barrel, upper or anything like that, this was just an example question I was spit balling).

Anyway, I thank you all for any thoughts you may provide.

Kyle

markm
10-11-21, 17:34
They still make 6.8??? :stop:

454308
10-11-21, 18:12
They still make 6.8??? :stop:Still more useful than 300 worthless.

Sent from my SM-G973U using Tapatalk

Apoptotic
10-11-21, 18:24
I don’t think this company has since 2017/18.

Deadman William
10-11-21, 18:52
i dont think youre at risk of any kind of long term damage.. if the bolt locked up and didnt blow up, unlocked and cycled and reloaded without issue, what damage could there be? the only malfunction you experienced were extraction problems while shooting fast - which, i think we can assume safely, is due to the fact that the base of a 6.8mm casing is significantly larger than the base of a 5.56 casing - really im just shocked it would extract at all

so we can pretty safely infer the dimensions of a 6.8 bolt (or at least the bolt you had) and a 5.56 bolt are identical except for the bolt face milling, which doesnt interact with the chamber/upper at all. if the bolt was a totally different spec, it would be different by a lot more than thousandths of an inch - otherwise, why not just use the 5.56 bolt and save engineering an entirely different bolt?

so my guess is your bolt was either mistamped and was actually a 5.56 and you were malfunctioning for some other reason, or the 6.8 bolt is a milled out 5.56. since i have no working knowledge of the 6.8 platform, i cant say for sure but i think the rest stands on reason

Apoptotic
10-11-21, 20:54
The bolt face definitely has a larger circumference. I appreciate the other info you provided as I now have a better understanding how the rifle even had a chance of firing and cycling as well as it has. Also, I have a much better idea of how the expended brass was flipped nose up and jammed in between the bolt face and the charging handle (the rim of case had wiggle room when the extractor had a hold of it). Many thanks.

titsonritz
10-11-21, 21:21
so we can pretty safely infer the dimensions of a 6.8 bolt (or at least the bolt you had) and a 5.56 bolt are identical except for the bolt face milling, which doesnt interact with the chamber/upper at all. if the bolt was a totally different spec, it would be different by a lot more than thousandths of an inch - otherwise, why not just use the 5.56 bolt and save engineering an entirely different bolt?



No, we can't safely infer that, 6.8's has a rim dia of 0.422", 5.56 has a rim dia of 0.378".

Deadman William
10-11-21, 21:58
No, we can't safely infer that, 6.8's has a rim dia of 0.422", 5.56 has a rim dia of 0.378".

i pointed out the rim diameter difference. how can we not assume a 6.8 bolt fits into a 5.56 barrel extension if it does?

AndyLate
10-12-21, 08:08
i dont think youre at risk of any kind of long term damage.. if the bolt locked up and didnt blow up, unlocked and cycled and reloaded without issue, what damage could there be? the only malfunction you experienced were extraction problems while shooting fast - which, i think we can assume safely, is due to the fact that the base of a 6.8mm casing is significantly larger than the base of a 5.56 casing - really im just shocked it would extract at all
~SNIP~


This. It is a very interesting scenario, but I would not expect any unseen damage.

Andy

GH41
10-12-21, 08:54
Twice you said it stove piped between the bolt face and charging hand... Is this rifle a side charger?

Apoptotic
10-12-21, 09:45
Nope, it’s a more or less Milspec rifle.

Clint
10-12-21, 19:04
Ejection issues would be pretty much expected, as the extractor can barely get a grip on the rim due to the diameter difference.

Replacing the bolt should fix it up and there shouldn't be any concern for any undue wear or damage.

Apoptotic
10-13-21, 03:06
Much appreciated, Clint.