Roller nut materials: There are plenty. For cost and efficiency, I normally use Delrin (polyethelyne) for the nut, 1 and 3/8 or 1 and 1/2 in diameter will work fine, if you put in a steel plug for the trigger to work against. The best quality medieval hunting bows usually use antler (not cow-horn) I find Moose-stems work very well. Axis stag is good too if you can get bits thick enough. Elk horn tends to be very pithy in the middle, so doesn't make a good roller. If you're working at less than 100 lb. a good hardwood like maple or oak will give years of good service (again, you need a hard point, brass or steel for the trigger sear) My very first crossbow used a brass nut. Its fairly easy to work and looks snazzy, but the stuff is very heavy... the slow turning will contribute to misfires, though drilling a few lightening holes should help. A solid brass or steel nut probably won't need a reinforced hard-point for the sear. If you're working from Payne Gallwey's plans, I recommend you make your nut socket a bit deeper than specified. He sez to sink it 3/5 of diameter, but 2/3 is more secure. Have fun with your build. Geezer.