im not 100% but i think a nipple is what goes into the breech plug to stop powder from falling out during loading. and after u load it u pull it out and put a primer in. im new to ml too but i know quite a bit. if u have a question make a post and name it "for cayugad" this guy knows EVERYTHING about ml. also i think if u dont use powder and u use pellets u dont need a nipple