As does Miller, I'd very much like to hear from someone who understands this stuff.

I did get some help from Adam at METL who is a metallurgical engineer and not a mechanical engineer.

Barlow's refers to a pipe capped at both ends with a static pressure. The barrels aren't designed to be pressure vessels as one end is open. This is why an obstruction could cause a rupture at a pressure below proof pressure. 12,205 psi may be the right value for stress the barrel can sustain if it were capped at both ends.

https://sites.google.com/a/damascusknowledge.com/www/home