That much powder would not be fun to shoot. As these guns were designed for round ball (IIRC), I'd think up to about 120gr. might be all that will fit the case - in the Husky. I was shooting only 3 drams in my 13 bore Husky, about 82gr.FFG (real BP).

Due to the rifling depth, a smaller ball and heavy cloth patch might be the best approach. I say this due to the depth of rifling and the very real possibility of severe lead fouling with a groove size ball.

As to the 12.7x44R which is very similar (the same?) to a .50/70 - the appropriate bullet would be 350gr. to 450gr. and around 70gr. FFG.
There might be more info on the Swedish Forum at "Gun Boards"