Used to be common--my screen name was originally a pistol: A Velo Dog was a small handgun made for cyclists to carry to protect themselves from bands of wild or feral dogs. Do a Google images search and check it out.
As for carrying a rifle, why couldn't you attach (duct tape?) a cheap scabbard to the down tube? Only rufle I have around is am M-1 carbine, but it's about the right length to fit comfortably there, and you'd have reasonably good access.