Rti prophet .30 talk

took my .30 cal prophet out today, only had slugs to use, shots were all over the place, not even worth photographing, very impressed with how powerful the little gun can be, if I remember correctly i had hit 90 ftlbs not even maxed on reg. Does anyone know how i can improve barrel to shoot slugs? Maybe even give me recommendations on what works out if it, thanks again guys- ps i was trying the nsa 43 gr and 47 gr