My suggestion, cut the barrels to 18.5" as described by Cajunlawyer, play with it for a while in that configuration. Its easier to handle and control a short shotty than a really long 12 bore pistol, besides, when you shoulder it you can hit stuff, pistol style, not so much.. I've seen some real abortions show up to shoot a match( Mossberg intimidator, vindicator or molester, or sumsuch) and the user usually goes home bruised or bloody. Never try to shoot a pistol grip buckshot load over the hood of a M-715 when your schnozz is ANY where near the back of the gun. I wish I'd had a camera running.
Any how, If you just have to, make absolutely sure of the final measurements, remember Ruby Ridge?? Probably started with the same question, or intent. I have a 23" SKB SxS and its handy, plus I can shoot skeet with it rather well..
suggestions off.