A ball-peen hammer is *the* tool when you need the ball end, but any hammer will serve for tightening the butt cap. It's just a gentle (read slow) spreading if the very very tip of the tang where it comes through the cap. A slow tap, tap, tap will spread the tip and tighten things up, then a careful filing to remove the excess tang tip material. No clamping, glue or duct tape required. Just a gentle hand, small file and not being in a rush.