Having shot Cowboy Action for over 16 years and using multiple length barrel shotguns, I can honestly say have it cut to 18 1/2" if this is the gun you plan to use regularly. Keep in mind you will have absolutely NO choke so knock down targets past 20 meters will be a challenge. I used to use a 30" Winchester M97 until I spent a horrible time one match trying to get around multiple props that were put in the way. The barrel hit everything. I had my barrel cut to 18 1/2" (legal to take to the US for matches) then had it fitted with screw in chokes. Some non SASS affiliated clubs put their targets a little far so I just screw in the full choke to my barrel and I'm good to go.
Keep in mind, very few shooters these days use cap & ball revolvers, if during a match you have to "load on the clock" you are at a decided disadvantage! SASS affiliated club matches are one thing, but some non affiliated clubs have really wild ideas at times!