Out of the box the MC312 runs 12GA 23/4 3 and 3.5. We even tested target loads 12GA 23/4 and hot 3.5 mixed in the same mag tube and the shotgun still functioned. This is a very difficult task for most shotguns as it requires the bolt to cycle back far enough to load the 12GA 3.5 but using only the energy from a target load 12GA 23/4. We really abused this gun in testing. We ran 700 rounds continuous leaving no time to cool we mixed and matched ammo in a single mag tube and we put it through some of the most grueling tests we could come up with. We ran the first 5000 rounds with zero cleaning then ran a 10000 round endurance test without the replacing any parts. For the last test we dissassembled 5 shotguns placed the parts in a box mixed the parts up and reassembled the guns with a random selection. The MC312 fired with NO ADJUSTMENT NEEDED. - The GIRSAN MC312 even comes with a limited lifetime warranty. NO ONE puts a lifetime warranty on a shotgun EXCEPT GIRSAN and EAA CORP. - Gauge: 12GA - Barrel Length: 28 - Choke: SCREW - Overall Length: 50 - Length of Pull: 14.25 - Width: 1.5 - Type: INERTIA SHOTGUN