Would a 6BR upper for an AR15 be possible?

If so what kind of velocity could be expected out of a 20” barrel shooting the 55NBT, 95NBT or 105Amax?

How would you get the bolt face to .480"

I would assume you would mill out the bolt face. I have seen pics of them milled out for the WSSM dia so I would assume the BR case is doable. How long the bolts would last, I don’t know?

I think it is a bad idea. When you look at the tenon diameter, and then subtract the chamber diameter at the head, you just don't have much metal left. At least not enough for me to put 50,000 psi to it. I know guys do it, but that does not make it wise.

Understood.

These short mag and other fat cases on a standard AR-15 barrel extension are just a cold weather accident waiting to happen.