Rather than use a 3 inch and lose the recovery time from the additional recoil, I went to #1Buck.My choice is a 12 ga coach gun with 3" chambers. I feed it 3" 00 buckshot with each shell holding 15 ( not 9 ) 00 buckshot. it also holds 5 more rounds in a side saddle bandolier. It uses dual triggers so I don't have to worry about a single trigger inertia mechanism failing to swap barrels when a shot is fired. The coach gun is the partner of a .45 acp 1911a1.
That's 16 or 20- depending on brand and whether they are plated or not- .30 ball every shot. 00 Buck is .32, so only you and the coroner would know the difference.