Ok I have a Noveske KX3 in hand. I'm giving thought to just going with a non NFA set up on this build. So to get to 16" With the KX3 spot welded what length barrel will do that?? My 3rd grade math is telling me 12.75. But how do they measure a stripped barrel?? Is it from the muzzle end of the treads back to the barrel ext. Or where the bolt stops to the end of the muzzle threads???

it would be from the bolt face to the end of the threads for the actual barrel length.  your safest bet would be to tell the gunsmith exactly what you want to do and you need it to be ~16.1" total when complete.

for an example, a 14.7" barrel + crush washer + a2 flash hider is ~16.1" when installed.  you could figure the difference between the length of an a2 flash hider and your kx3 and subtract that from your barrel length.

definitely a time to measure twice, cut once.

13.7 with a KX3 is something like 16.2ish. Others have said that a 12.5 with a pig will not get to 16.