My old M77V Swift had a 26" barrel. You sure you brother's is 24"?
Seriously doubt you will get good results with anything heavier than 55 grain bullets. But that's fine because with a good bullet you set for deer and peccary.
Might try either RL-15 or IMR 4064 with 55 grain bullet. 38 grains of either one. I have killed many a critter with those two powders. There are lots of excellent powders as well as bullets for the .220 Swift and it should not be hard to find something that works well for you.
