You dad is right...Conical style bullets didn't come around until the mid 1800s...By this time the deer population had been practically wiped out by hide hunters using round balls in flintlocks...

I've killed dozens of deer with a flintlock, most with a .45 and a .54...They work as well as anything when used within open sight range...

If he hits a deer in the right place, it won't go far, same as any other projectile...