How shells are loaded has little to do with chokes. The same principles apply whether building long distance goose loads. Although I seriously do not think an IM choke is too loose for 21 yards, you can put more pellets on target by using high quality plated shot, buffer the load, and use un-slit or barely slit shotcups. If you can find some international trap loads loaded with nickel plated shot that is probably going to be the best you will find, loaded. They ain't cheap.
Or, far easier, you could have choke tubes fitted to your barrel. Just make sure you get a good gunsmith to do it. Un-screwing up a barrel costs even more.