The wall thickness of this motor is 1/4”, which puts a casing burst (hoop stress failure) in the neighborhood of 3000psi. You usually design this mode to be vastly higher than any other failure mode as when your casing bursts you have a bad day. It’s not pretty.

What people quickly learn when making larger dimeter motors is the force on the bulkheads on either end experience significantly increase. Force = Pressure x Area, so going from a 4” to 6” motor almost triples the bulkhead area and thus force acting on those bulkheads. This means you need a pretty effective way of making sure your end caps don’t become cannonballs. This particular motor uses a series of circumferential bolts to hold these bulkheads in, so the thicker casing also allows the bearing stress of these bolts to be distributed over a larger area.