I saw a 20 gauge lawfully shortened to 22 inches before, leaving no choke. I was surprised at how good the pattern was, so long as high base, expensive shells were used. These types of shells have a good shot cup and wad in them, which serves to hold the shot together for a pretty good distance.
It looked like a good 30 yard pattern to me, akin to a skeet choke.
Mannyrock