Congratulations on the all-out decision to try round balls - with a slight roll crimp over the ball, I think the 2 1/2" - actually 2.4", will probably do just fine. I use straight walled plastic for my 12 bore, ball loads, which shoot well in the .714" bored (.724" groove) Mosberg pump. I used a cut-off steel shot wad over the powder. The top surface where I cut off the fingers had a nice cup shape, while the bottom cup against the powder had a squarish cup. Seems to me, ai had to add a 1/2" and card wad beneath that cup, rather than using the gas check off another wad.

The interior capacity if the brass is considerably more than plastci due to thinner walls.

Plastic wads will most likely be too small as I have to use 14 bore wads just to fill 16 bore MagTec cases.

Please don't use the phoney black powders. They contain chlorates which are corrosive - 17% of the charge, compared to corrosive primers of years gone by in which the chlorate, making them corrosive, was miniscule instead.

You can use data from Lyman's shotshell loading books - pick the ones that produce low pressure and reduce a few gr. to start. I plich the lowest charge they list, which is usually for a one piece tight plastic wad, then develope my own data from that - or - I pick a shotshell load for similar weight in shot and go from there. I use SR7625 and SR4756 mostly, but have also used Herco which is just slightly slower than Unique.

A ball diameter that is just barely smaller than the choked section will be a good place to start. "Track of the Wolf" sells balls in many different sizes, in 25-count bags. they will be pure lead. In my own 12 bore with .725" bores, a .684" ball was very accurate due to the cupped wad beneath it, holding it centred in the bore.