If it were me, and you can find it, I would probably shoot 6s instead of 5s in the 20 gauge. Personal opinion, but there are quite a few more 6s than 5s in the little 20 gauge shell. I would start with the .565 constriction choke with either size shot. I have a .565 I use in my 21" barrel, and I normally shoot the Federal TSS, but I have also tried both #5 and #6 LongBeard XR in the little gun. The 6s consistently out pattered the 5s and would be good to 40 yards. Each gun is different so have fun