Inspection on receipt of parts by volume assemblers means you can size the uppers and extensions, batch them, then use the complementary ones that fit. A large enough builder doesn't need to pour goo on the extension to make it fit properly - he has enough to pick and choose the right one.
Rocksett the barrel = extra fee = more profit.
You want more accuracy than that, shoot the barrel to find the group center, rotate to twelve o'clock, then pin the barrel extension, cut the ramps and put the gas port in. Now the barrel is vertically oriented with the dispersion in a direction you can use rather than pointing sideways enough to run out of windage on the scope.
Yes, that does happen because we don't know where the barrel will shoot. We just take our chances. Why when it can be determined and fixed correctly rather than pouring goo on the extension and still being stuck with the problem?
Fooey on goo or squaring the nose, get the barrel to group at 12 oclock and I'd pay another $50 for that. You'd be buying value adding labor and accuracy, not a rifle that could never be altered or repaired.
Smells like somebody is selling something.