I've been on training sessions that just used ordinary Walmart or CTC bulk crap birdshot shells for practicing various manoeuvers. There's no need to use the 'real thing' until you specifically want to figure out patterning and effective range issues, mostly you just want a big bang and recoil.
Personally, I really dislike 3" or larger shells in a tactical operation. Except in a double gun, you're limiting the number of rounds you can have in your magazine, and that's where you want to have them. And for hunting, I strongly suspect that they overpower the shot column and wreck the patterning you want for wing shooting.