Vegard_dino - first thing to do is to measure the smallest diameter of the bores - the choke to decide what ball diameter is required.
Here are shot shot loads, patched round ball loads and round ball loads for my 16 bore - all loaded with black powder in 2 1/2" brass due to the 2 5/8" chamber. Using plastic in a 2 3/4" chamber, a roll or folded crimp can be used with round balls - just depends on the wad clumn height.