Hi all, I'm purchasing a brand new Bushmaster HBAR that a gunsmith has rebarreled with a Colt govt profile A2 barrel.  The smith test fired 10 rds after rebarreling it.  First 3 didnt fully eject, this was said due to "carbon needed to seal".  Then next 7 rounds functioned perfectly without problem to the case.

I'm wondering if this "carbon need to seal" is part of normal break in after a new barrel's in place, or if it's something else.  Is it a common thing you should expect after rebarreling an AR?

By the way, if the AWB sunsets, would it be possible to remove a pinned Cav compensator off the barrel?  Thanks in advance.

He was probably talking about the gap between the front sight/gas block and the barrel.

Also between gas tube and gas block.

yeah I kinda guess that must be it.  There really shouldn't be that much gas that left thru the gap to cause problem in extraction though.  any ideas?

That's gunsmith-speak for "Gee, I sure don't know why that happened!"

Never heard of it.  I built 4 new uppers to date with no such start up problem.  May be just new parts needed to mesh.  As long as it's working now, have fun shooting.

There it is!

Snow job.  Test for proper functioning by placing one round in mag, fire gun, see if the bolt is locked in the open postion.  Do this for 20 rounds or so.  If it fails to lock open once, I say you have a "short stoke" problem.  I test all my rifle with 50 rounds with the gun held loose without shoulder support.  Mine lock back everytime.

Well that's just it, I can't test it as I'm going to buy it, but that's what the seller said about the gun's history.  It appears that gun smith tested 3 rounds that failed, then 10 that went great, so he stopped there.  The rifle hasn't been fired since.  I wonder if I should bother now.  

I didn't think it'd take some carbon sealing, if there's leaky gas, then no carbon can seal it, it's not like a car engine.  What would be the cause of short stroking?  Thanks.

So is this something common to newly built barrel and gas block?

I have never heard of this before.
A new gas system either works or it doesn't.
What may have happened is that the parts were a little rough from the milling process, and it took a few rounds to knock the burrs and spurs off.
I personaly agree with Russ4777.