I suppose it depends what you are going to be shooting as to what gage you need, ie a 410 in my opinion is a waste for pigeon shooting but great for ratting.
I ain't no technical expert but moderators work most effectively if they are used in conjunction with sub-sonic ammo that travels at less than the speed of sound ie 350 meters per second (1,148 ft/s). This equates to 1,270 kilometers per hour (789 mph), or about one kilometer in three seconds and about one mile in five seconds. I used 32gram hushpower subs in my 12 bore.