The Baikal/Remington Spartan coach guns are also 20 inch cylinder bore, except the version that has removable choke tubes. Most others that I know of, like Stoeger and CZ, are usually choked IC/Mod, I believe. I wouldn't cut off more than 1.5 inches myself. Not much, if any choke, would be left and you'd still be legal. I remember looking at a used Brno coach gun that had barrels just over 18" and a slight cast to the stock. It was a sweet little gun.
Oh, BTW, Baikal does/did offer a version with functioning external hammers but it also came with 4 removable choke tubes.