I use a medium sized ball peen hammer, and start with the round end, working around the outside edge of the rivet with light taps. When metal has begun to flow and the countersink is almost filled, I go to the smooth other face of the hammer, again working the outside of the rivet, and the few final taps are over the centre just to spread it a little, and make the surface rounded. Dress the rivet off flush with the nose cap, and any tiny divots can be filed out.