I have not loaded any 28ga black powder shells. However I believe a big problem with them is going to be one of "Space". My old Lyman shotshell handbook had a chart showing length of shot column for 1oz of shot for the different gauge sizes. For the 3/4oz load in a 28ga this will work out to about .907", so doubling that for equal volume of powder you have 1.814". Suptracting base wad height, wad column length & allowance for sufficient crimp, you aren't going to have a lot left for an increase. Of course you gain a little in that the internal case dimension is larger than the .550" nominal bore dia this was figured on & the powder charge will compress some in loading. I suspect though you aren't going to get much over a 2dram (55gr) - 3/4oz load in a 2½" 28ga case, with an efficient wad column. The brass case will have thinner walls, less base thickness & not as much length lost as is not crimped, so more capacity.
