As for ammo, I know of no US made bad stuff, as far as reliability goes. Each shotgun is a law unto itself as to what works best in it for patterning. Take two shotguns with consecutive serial numbers off the assembly line and test them, chances are each one "Likes" a different load.
While lower recoil loads like trap loads are a valuable part of any training session, no session's complete w/o shooting some duty loads,whatever they are. Rarely, there's an incompatability that should be discovered in practice and rectified, not in the midst of an AS scenario.
I use 00,simply because it was my duty load, its effectiveness is legendary and there's a lot of it still laying around the Casa. However, the first instructor school I was sent to was manned by some FBI types, and they did fine work in the demos with #4 buck,fine work indeed.
We tested some #1,for possible use in the towers,and liked it for the slightly thicker pattern,but bureaucratic inertia triumphed and the system stuck to 00.
And, there's something to be said for the old mix loading. In a pump, that means the first one or two up are smaller pellets, like #4-8 shot, with some heavier loads behind for better penetration through say, your refrigerator. There's both pros and cons, so think it over.
Pros, lesser penetration of walls and a lower chance of injury to a non combatant, family member,neighbor,etc.Lower risk of serious damage from a ricochet,too. This last is mayhap more theoretical than real, I've heard of few ricochet injuries in real AS scenarios.
Cons,less effectiveness at longer ranges, if the scenario moves outside, or you live in one BIG house.Also, there's skip shooting techniques that work best with buck, tho they tend to be special case apps.