I don't know what my pressures were, gatsby, but seemed OK in my turn of the century gun - no excessive flattening or pushing back the firing pin as happened in my 10 bore with factory ammo.

Lyman lists 12 bore loads up to 42.0gr. and 43.0gr. of SR4756 in 2 3/4" Fed plastics with a list of wad parts and a 475gr. Lyman slug at 1,500fps. Those are running 10,300 to 10,500LUP.

With a Brenneke slug and extra wads, roll crimp, 36.0gr. 4756 at 1,532fps and 33.5gr. Brenneke and fold crimp at 1,457fps. Those run 10,500LUP and 9,800LUP respectively.

With .690" round ball, slighlty larger than mine, 40.0gr. 4756 with wads including a WAA12 with petals removed + 1/2" fiber, 1/8" card and .690' RB, roll crimp, was 1,538fps.
36.0gr. .690" RB, WAA12 patals removed same was and fold crimp ran 1,456fps. The pressures were both 10,200
LUP.

All these loads are well under 12,000LUP, noted as being about maximum for shotgun shells.

I suspect when I loaded my round balls to 1,550fps, I might have been pushing around max - I had no trouble, however I no longer have that data - dagnabbit, so for my new Mossberg 835 rifled gun, I'm starting over again, but not without places to start.

I started with known shot loads - ie: 1 1/4ounce data and went up from there after chronographing the black powder loads, then duplicated them in what seemed to be a good stable slow burning powder. I seem now that Hodgdon's Longshot powder might be a good powder to try.

This data was all taken from Lyman's old 3rd Edition Lyman Shotshell Handbook. I'm sure they must have a newer version out now, perhaps several newer books.

This one has Herco loads along with Hi-Skor 800X, Blue Dot, Unique, 571 and SR7625, with various different projectiles, wads and cases. All of the 3" and 10 bore data seems to be buckshot. Perhaps the newer books have 3" and 3 1/2" data due to the newer guns, like my Mossey 3 1/2".

Ed Huble has taken this into the next world with the use of H4227 and H117 powders along with Fed 239/439??? primers, achieving incredible velocities out of what seem to be normal SXS and pump shotguns. Of course, his loads "from hell" are for re-barreled, heavy barreled guns, rifled and smooth using 'modern' brass cases. For example, a 100gr. bullet at 3,400fps. That's over 25,000fpe.