I saw someone the other day who had mounted a tube on the side of his back rack pointed at the ground and secured somewhat like a pannier, with a rifle butt sticking out of it secured with shock cord in some manner. I didn't pay too much attention to it, since I don't need to carry a rifle anywhere myself.
Also: Don't see the point in carrying a very small firearm personally - lots of people carry guns here, but they're hand cannons or rifles because they don't expect to shoot it at anything weighing less than 400 pounds.