as the saying goes ' if it aint broke dont fix it'
u can use what u would like to get the job done. some have use a hole saw withe teeth ground off and shrpened, others have used a round punch or half round aswell. with copper it may dull often, ie. a copper pipe maybe too soft but u could put a plug of lead or something then hammer on it. a right dia. pipe of any steel would be good and if it fits a drill press that works better.