With a 3inch 12g chamber you can buy a box of regular 2 and 3/4. Not sure what you plan on shooting at though. 4 is a little big for anything other than patterning and hunting medium game. Yes you can pattern any thing you want. They have a couple places for you pattern shot guns here.
Call and ask. They do have shotgun range.
Yes you can.
Yes
If you want to shoot clay pigeons must use 7&1/2 or smaller. You can pattern at the all purpose ranges out to 25 yards which is little close, usually pattern at 40 yards.
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