I've used 16 oz leather mallets, 3 lb. dead blow hammers, 1 lb brass hammers, 5 lb lead hammers and of course a yellar plastic hammer and it does not make any difference when seating primers with a Lee Loader. Just make sure you watch what you are doing, make sure the primer is straight in the "pocket" of the tool and the spring loaded washer is free and you'll be fine. I had only one Lee Loader that would pop primers more than one out of 200, that was my .44 Mag loader. I just chamfered the primer pockets ala removing military primer crimps and haven't had a primer pop since. I have an arbor press in my shop and that makes primer seating with a Lee Loader a breeze, and easier on the hands than a hand primer...