How common is a cracked gas cylinder?

[Snafu]

Mine cracked from leading edge across hole and it is 1/2 way back
Its now out for rebarrel

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[Bill Hollinger]

I don't think that is all that common. It happens no doubt though. I have never seen one personally and I have had a grip of carbines over the years.

[Bubba-7]

From my experence it is common on IBM barrels. I have seem others cracked to the drill hole but only IBM barrels cracked beyond the hole. I have seem one that was repaired and did not hold. A new barrel is the only long term fix. I would assume if the nut was welded in it would reinforce the housing enough to last. But money spent to do that would go a long way toward another barrel.

[landtrain]

Not common but it's usually caused by trying to remove the plug that has been staked extra hard. This was usually a depot maintenance job. TIG the crack and re-chase the threads. Tighten the plug then test fire a few rounds. Look for carbon stains. If present the weld didn't hold. If no stains, then re-stake the nut and reinspect every mag fired.

[Snafu]

From my experence it is common on IBM barrels. I have seem others cracked to the drill hole but only IBM barrels cracked beyond the hole. I have seem one that was repaired and did not hold. A new barrel is the only long term fix. I would assume if the nut was welded in it would reinforce the housing enough to last. But money spent to do that would go a long way toward another barrel.

It was an Underwood on a NPM

[INLAND44]

Even correctly welded and safe to shoot, the carbine is greatly diminished in value. I would speculate that most cracked gas lugs are from overtightening the piston nut or trying to force it when cross-threaded. As painful as it may be, a re-barrel is the thing to do. I feel safe in adding that I have never heard of one cracking from firing, so if it's not cracked and you DON'T UNSCREW THE NUT you probably won't have any trouble with it.
I think it's remarkable that an expendable small arm, hurriedly developed in the darkest days of the war and used in three major conflicts, is still existing in such large numbers over sixty years later and with so few problems.