I am somewhat smitten with the very tiny sized RMR by Trijicon. I have yet to mount one on a combi rifle, but I have them mounted on double rifles. I have tried both the battery version as well as the fibre optic illuminated variety. I like the tiny but bright battery 3.5moa self adjusting brightness dot and find it extremely fast to come up,on target, but batteries do occasionally run flat, and I have also found that I need to make sure that the battery contact has enough bend in it to ensure positive contact with the battery terminals - flat CR 2032 type watch battery. The version that runs on ambient light without aid of a battery with the triangular aim point also seems to work well, but if one is standing in a darker environment (blind, etc) and aiming out into bright sunlight, the reticle can be very washed out and difficult to see quickly.

In my humble opinion, the RMRs are not without fault, but they are indeed very small and unobtrusive, and do work much better than iron sights for my aging eyes. Trijicon also offers an unfitted claw type base for the RMR, which I am considering buying and then spending some quality time working the mount with a file to try to fit it to the rear claw base of an old 7x57R, 16 gauge 2 1/2" combi gun I have.