I use SuperHandicap 7 1/2's for the first shot, Federal Premiums second, unless I end up in a shootoff and back at 36 yards or so, then the Premiums for both.
If an O/U, I use I/M and Full, in that gun is .030" and .040" but I make sure they do that on paper, and they do. After you slide (a term for running 5 birds and moving back a yard) a few times and end up at 34 or so, or in a shootoff and moving back, put all you have in that you know patterns well, a pair of .040's will work.
A quick note, if a bird comes out of the trap dead, and they do on occasion, you put a shot somewhere in the direction of that bird before he hits the ground and it will be scored dead.
At least here, only lead shot is legal. It can be plated though, 1 1/4 max, any velocity.
Gamebore makes a shell called "Buffalo", 1 1/4, that goes out at 1450 and if you connect with that puppy, it will skin 'em and fry 'em.