Yes it is actually a law that you can't put firearms, or any explosive device on an RC aircraft! Yes it has been done. One factor to take into consideration is the recoil of the round to be fired. Recoil from a 45 derringer will totally disintegrate a 6# 58" plane with a 58" wingspan.
I've flown RC since the mid '70s. Have a plane with a 16foot wing that has since been relegated to hanging from the ceiling of the hanger, that we used to drop everything up to and including 16# bowling balls from, sometimes with a utility chute and sometimes without just to see the end result. The plane was originally built in the early '80s to be a part of a celebration dealing with the advent of airmail and was flown nonstop over 150 miles with a load of mail on board with the pilot riding in the back of a convertable on the highway below.