Barrel Extension Separation!

[mdram]

I've installed I purchased an 11" 6.5 Grendel Criterion barrel (NFA) from a well known vendor that bores their own barrels but apparently outsources the nitride finish.

this part confuses me
if the barrel was bored by the vendor, then its not a criterion
now if the vendor bought the blank, profiled, installed the extension and chambered, that is different.
boring is creating the bore and rifling.

and please do name the vendor