I guess a guy could use what he wanted to in his own rifle - i'ts his gun. BUT . . . that being said, I guess I could also shoot nails, rocks and cow manure as well.
IF, they happened to fit - you say they have rifling marks - is it going to match the rifling in your rifle? i doubt it. How would they react to the twist in your barrel, IF you could get 'em down? Are they lead or FMJ? There are projectiles designed for muzzleloading . . . and there are projectiles designed for modern rifles and handguns. "Nary the two should intertwine."
A big +1 to what Hawg said . . .
I have retrieved lead from backstops that were fired from muzzleloaders . . . but washed it well, melted it down, fluxed it and skimmed the impurities off and then cast it into new rounds - all soft lead. As far as picking up rounds and recycling 'em as they are . . . to me that's akin to picking up someone's cigarette butt and smoking it. Personally, I value the bore of my rifles and smoothbores too much . . .they only eat "new" projectiles.
If a pair of '51 Navies were good enough for Billy Hickok, then a single Navy on my right hip is good enough for me . . . besides . . . I'm probably only half as good as he was anyways. Hiram's Rangers Badge #63