I got a 12 gauge rifled barrel for my Maverick 88 (poor mans Mossberg 500).
It is 24" and i "crunch" numbers (qualified WAG) with QL. I strive for lower than 1400bar~20300psi pressure and always adjust load column for 0% airspace. I know not perfect but it will get the ballpark and i have yet to see any deformation of caseheads!

So my question is:
How far have you got?
I'm in the ballpark of 9kj~6.7kfpe with a 690gr modified Lyman Foster slug (630m/s~2050fps). No pressure signs!

READ 12ga FH thread on here..

What cases are you using and do you roll crimp, And
is the Lyman slug full bore or one you use in wadcup.Ed

A friend of mine recently took a couple of buffalo with a 12g firing round lead ball at around 1300fps. Very effective, he recovered a few lead balls from under the skin on the other side of the buffalo. It kills very effectively.