Im No Fan of Powerbelts, but try upping the charge to al least 70-80gr's and see if it tightens up. I would consider trying a 10mm 200gr XTP and about 70gr's of Pyro and see if that helps. It a very accurate bullet out f a CVA with minimal recoil and with the low Powder Charge it should do the job just fine out to 100yds.
Scrap the Powerbelts and do You and your Son a Favor.
(BP)