If the gun is a higher bore, then the inside of the barrel is narrower, so the cartridge it fires is smaller. Hence an 8 bore is a monstrosity of a gun, whereas a 28 bore is very small. 12 is by far the most common, but 20 bore is common also. I have heard of folk using 28s for game but don't know anyone myself who has one, nor have I ever had a 28 so could'nt speak of what they're like practically. Also there is the .410 which is tiny, ok for ratting and rabbiting, but not a serious game gun (I'm assuming you're interested mostly in game shooting as you posted in driven and walked up). Person