Originally Posted by Zak Smith
I can't say why they cut their threads that way, but one reasonable explanation might be that 99.9% of their customers use a muzzle device (flash hider or muzzle brake) which is timed used a crush or peel washer. Not making the root cut at the back of the threads is one less machine op during manufacturing.
I see a lot of factory barrels of all manufacturers through my shop for service work and probably 95% of them do not have a clearance cut behind the threads.
Only custom threaded barrels get the clearance cut.
This is because most gunsmiths use a manual lathe and you need the clearance to start or stop the threading tool against the barrel shoulder.
On a factory produced barrel made on a CNC machine, the machine does not need to make a relief cut behind the threads as it can just get the threading tool out of the way on its own.
In my opinion, any 1/2-28 thread direct mount suppressor should have a 1/8" counterbore to accommodate all factory barrels.
That way, this situation can never occur.
Every suppressor QD mount I have seen has had the cut so that the mount can be tightened all the way against the barrel shoulder.