There are so many shotguns that will work for NCOWS. That is why we say “too numerous to list completely”. There is no list. Any shotgun design that is pre-1899 will probably work. This includes single barrel, double barrel (not O/U) with two or one trigger. Exposed or internal hammers. Period pump shotguns. The most common ones that will be seen is the Winchester 1897 (and clones), Stoeger Coach or Upland, and reproductions of the Colt 1878 SxS.
Baltimore Ed, the Marlin 1898 Pump shotgun is OK, but they do tend to shoot loose. You would have to prove to a local judge (or me) that is is solid and safe.
If anyone has a question on acceptability of a particular shotgun, email me at
judge@NCOWS.org. I will give you an answer.