There are some shortened Win Mod 12's around. There were two on the shotgun for sale section of this site recently. I believe was cut down and the other is a USGI trench gun from previous wars.
US govt. used many models for military use Stevens, Savage, Winchester, Remington, Ithaca, etc. The Model 12 is a fine gun and would be nice to use this way. It has no trigger disconnector so each time you pump the gun it will fire if you are holding the trigger back.
I have not seen any short barrels for 12's for sale so I guess your option is to buy a longer one if you can find one and have it cut down or cut down your existing one.