had this same prob. put standard recoil spring in stock, lightened bolt up by 35%. can do 15/18 splits now & gun is waiting on me. use 3 1/4 dram 1 1/8 shot.
for my personal gun [ and most of them I build] I like a 5" slide,short 3 port comp, 3 gas ports in the barrel/slide. I lighten the front of the gun & slide so it transtitions very fast. gun weighs 41 oz. slide weighs 11oz. very flat shooting
you are limp shouldering the gun & causeing it to short stroke while standing but not the bags were you were leaning into it. try leaning into the gun & pull it into your shoulder.