I have two original longrifles with copper rivets holding the nosecap on, so unless you are replicating a specific gunsmiths style, you should be good Togo with a copper rivet.
I’ve used copper harness rivets with the head spun down in a drill motor, with a file. And have used bare electrical ground wire annealed with a head hammered on one end, mostly for small rivets on pistols.
Hungry Horse