Otto - check out the data in the #3 and #4 Lyman Shotshell handbooks. As all 12 bore modern factory stuff, no matter what the shot load, is loaded to approximately 12,000PSI maximum pressure.
; You will find loads in the Lyman book showing only 10,000SPI that produces 1,500fps + with round balls and slugs weighing in the 1-1/8oz. range and a bit more.
; In your rifle's 2 1/2" case, you should easily make 1,350fps to 1,400fps. Go carefully for advance to what you are comfortable with. The original .577/450 rifle ammo in the same action made almost 30,000PSI - probably in the relm of 25,000PSI at least. It is a very strong action and only limited at this point by the thin walls of the barrel.
; For round balls, one need not use a groove diameter ball. If the cup wad is cut from a trap wad, field wad or steel-shot wad, then placed cup-up with the ball sitting on top. the wad will take the rifling, spinning the ball perfectly, then fall off at the muzzle. This worked very well for me in a smoothbore, but no reason it won't work in your rifle too.
: The reason I bring this up, is a .744" round ball will weight around 640gr. in pure lead, 620 or so in WW metal which is about as hard as you might want to shoot. I have such a mould, made by Jeff Tanner. My bro is now using that ball, patched in .030" denim in his Bess, a 10 bore.
; A couple friends of mine tried patching .684" round balls and loaded them in shotshells with excellent accuracy in their full choke duck guns.