My primary shell in a 12ga is 1 oz of 8's at 1225 fps...
I prefer a standard 2-3/4 dram load of 1-1/8 oz of #7-1/2s for ATA 16-yard events. It has more pellets and less recoil. For practice, just about anything that goes bang will work at 16-yards. YMMV. Perhaps BigJim's inertia gun wont's cycle a standard load, so he has to load heavies.