Got a friend who has a AR pistol.  He assembled the lower himself but bought a complete upper.  Like around a 9 inch barrel I think...maybe 7.5.   Not long after he got it he was shooting and the bolt stuck about 1 inch out of battery.  We he got it home and got the gun apart he found the gas tube had come loose and was kinda stuck to the gas key.  We determined the gas tube roll pin in the gas block either broke or worked itself out.  He replaced the pin.

Fast forward to yesterday.  We were down at the range shooting.  After several mags through his gun he calls me over and says look at this.  I walk over and his BCG is stuck about 1 inch out of battery...again.   Sure enough the gas tube roll pin is missing...again.  Luckily I had a spare roll pin and gave it to him to replace the missing one.

Has anyone heard of this happening?  Is something that other users are experiencing occasionally?  I'm assuming the gas block roll pin hole might be a tad too large.  But my friend said he has to bang the other one in.  He said he was gonna use some kind of locking liquid(JB Weld, etc) this time on both ends of the pin after installation.

I had a similar problem in the past thar bushmaster didnt fix when it was on their table. Tell your friend to get some thin piano wire and thread  it through the gas tube roll pin and around the fsb where it is situated. Give it a twist with some pliers and your done.

Never heard of this problem before.

Is the FSB pinned ?

7.5 or 9" barrel I assume it is a pistol length gas tube. Just spit balling here and without pics I have no idea but it sounds like to me it has a gas tube that is too long. Maybe it was a carbine length tube that was cut down and re-drilled for a pistol length gas system. It could be just long enough that the carrier key is slamming into the gas tube, that constant slamming of the gas tube is shearing the roll pin. Once the roll pin is broken and falls away there is nothing holding in the gas tube and it's now getting stuck in the carrier key.


ETA fixed spelling and to add.

If it has a low profile gas block make sure it is not butted up directly to the shoulder of the barrel. IIRC there should be a gap approx 1/32" or the thickness of the hand guard cap. Using a feeler gauge make sure there is the proper gap between the gas block and shoulder. This small amount of gap will ensure that carrier key is not over traveling into the gas tube.

^ that sounds right.

That all sounds plausible.  I asked the owner about these things.  He says the gas tube appeared to be the proper length and didn't think it was hitting the gas key too deep.  It's got a YHM railed gas block.  No pic on their website so must be an older model.  The upper was built by JSE Surplus.  This is pretty close to what the upper looks like:  http://www.jsesurplus.com/7bd1x9556gasblockcustomupper-1.aspx

In regards to the roll pin breaking due to forces against the gas tube...if that would be happening wouldn't you be able to see one or both sides of the roll pin still stuck in the roll pin holes of the gas block?  As it is now the pin was fully gone.  Couldn't find it where he was shooting.  And also if the gas tube was actually breaking the roll pin wouldn't the roll pin hole in the gas tube show some elongation due to the banging against the roll pin?  Which we didn't see.

It's possible that the roll pin hole is oversized. I use cotter pins, seriously. Insert and bend it around the base or flat against the side of the block. It might look a little strange but works like a charm.

In some cases you need that gap, it depends on the gas block being used and the location of the gas port in the barrel.

Is this pistol breaking the roll pin or is it being drive out?

Yeah that is definitely strange. If there is no sign of wear on the gas tube and the gas block pin hole looks to be fine and is also not showing excessive wear. The only other thing that I could think of is maybe with heat expansion the roll pin is sliding out. But I believe it would take a lot more than 60rds to get it hot enough to do that.  

If he uses an epoxy make sure that it's heat rated and won't turn into goo if it gets too hot. Or maybe finding a piece of drill stock cut it to fit then stake both sides of the gas block so it doesn't work its way out.