My Revere did the same thing. 18 ft/ibs with 10,34grain JSBs and only 14 ft/ibs with ZANs 20grain. However I turned on some more hammerspring and the +20grainers are doing 23 ft/ibs and yet the 10,34grainers do only 20 ft/ibs. BTW you have a great slugshooter. I shoot heavy ZANs all day...