I don't see why you can't get the 1500FPS load with good pressure. You just need a slow powder that won't smoke the cases. I have had good luck with IMR 4759 not 4756 in myn 12 gauge. I use 64.0 gr of IMR 4759 behind a 615gr hollow base for 1555FPS. I can't give you a starting load for the 20, but Tom Armbrust can. He teted my load at 11,400PSI and 1555FPS average. Do not use H110 or 296 as they can hang fire, and explode! I was trying AA #9, but didn't have great luck with it. Ballpowders will give you more power than the slow flake type powders for the slugs. Try double base powders, and call Tom Armbrust to discuss your loads. He can be reached at 815-385-0037. Good luck Greg S
