Short stroking caused by BCG

[Captains1911]

I have a new 14.5” mid length upper that won’t lock back on the last round with a new Toolcraft BCG. I’ve tested it with multiple ammo types, mags, buffer weights, buffer springs, and BCGs, and concluded that the Toolcraft BCG is indeed the culprit. However, the carrier key doesn’t look or feel loose and is staked properly, and the bolt gas rings appear in good order and fit snugly inside the carrier. I’ve never had this issue before and am just curious on what specifically about the BCG could be creating this issue.

[dylank0723]

I had a similar issue with a triarc not long ago, narrowed it down to the bolt itself, I replaced the extractor and ejector springs and it still short stroked, swapped it with a colt bolt and it’s ran like a top ever since from 77gr all the way to wolf, not sure exactly what my issue was either


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[TomMcC]

Does the bolt move freely in the carrier? Does the BCG move freely in the receiver?

[Captains1911]

Does the bolt move freely in the carrier? Does the BCG move freely in the receiver?

Yes, everything feels normal in that regard. I should also note that it’s a NiB coated carrier, and well lubed.

[TomMcC]

Do you have another AR you can try the suspect BCG in?

[Clint]

This can occur with barrels that are very conservatively gassed or setup for mixed suppressed use.

The actual issue is the BCG is out of spec, usually in the bolt tail interface or gas key bore.

This causes extra gas leakage and reduced cycling power.

Non mil-spec coatings are a yellow flag in this case and often contribute to improper finished dimensions.

"Value"' priced commercial only BCG suppliers tend not to do a good job holding these critical dimensions with any sort of regularity.

The same leaky BCG may tend to run without apparent issue in a common overgassed barrel. This doesnt mean all is good, but a case of one issue masking another.

Swap the BCG for a high quality actual mil-spec unit and the upper will likely run fine.

[opngrnd]

This can occur with barrels that are very conservatively gassed or setup for mixed suppressed use.

The actual issue is the BCG is out of spec, usually in the bolt tail interface or gas key bore.

This causes extra gas leakage and reduced cycling power.

Non mil-spec coatings are a yellow flag in this case and often contribute to improper finished dimensions.

"Value"' priced commercial only BCG suppliers tend not to do a good job holding these critical dimensions with any sort of regularity.

The same leaky BCG may tend to run without apparent issue in a common overgassed barrel. This doesnt mean all is good, but a case of one issue masking another.

Swap the BCG for a high quality actual mil-spec unit and the upper will likely run fine.

I had this exact issue with a buddy. His S&W Sport was magically cured when the bcg was replaced. He tried new extractor springs, gas rings, etc.

[1168]

I helped a buddy fix a rifle recently that had this problem. In his case it was the carrier key. It didn’t feel loose, but one of the screws tightened easily when tested lightly with a hex key. I replaced the key and screws and used sealant and it ran.

[Captains1911]

Do you have another AR you can try the suspect BCG in?

Yes it ran fine in one other AR that I tested it in, but that AR is just gassed higher, so it doesn’t really prove anything. The fact that another BCG cycles properly in the 14.5” upper, is a sure sign that it’s the BCG.

[Captains1911]

This can occur with barrels that are very conservatively gassed or setup for mixed suppressed use.

The actual issue is the BCG is out of spec, usually in the bolt tail interface or gas key bore.

This causes extra gas leakage and reduced cycling power.

Non mil-spec coatings are a yellow flag in this case and often contribute to improper finished dimensions.

"Value"' priced commercial only BCG suppliers tend not to do a good job holding these critical dimensions with any sort of regularity.

The same leaky BCG may tend to run without apparent issue in a common overgassed barrel. This doesnt mean all is good, but a case of one issue masking another.

Swap the BCG for a high quality actual mil-spec unit and the upper will likely run fine.

Makes sense, thanks for the info.