I have a 1187 Super Mag w/ a 23" barrel and I shot a GT .655. I shoot both 3.5" Winchester Super X 2 1/4oz #6 shot and 3.5" Winchester Xtended #6 shot and I get great patterns. I get an average of 150-160 pellets in 10" circle at 40 yards w/ both loads. It is hard to say what constriction will work the best for yourgun. Just pick one and try it.The .655 has not done me wrong. I have tried the Jellyhead and a .660 Rhino and the GT .655 does the best for me.

Good luck and shoot straight!