I am gathering some bits to mount a J Point on my 12 ga 870 that has a 18 1/2" barrel on it. I'm using the J Point as I've had it laying aournd for a while.
If I didn't already have it, I'd most likely consider using a Burris Fastfire, as it is priced reasonably, is compact and comes with several mounts/shims. A Vortex Sparc is another option.
If you've got more spare $$$$ a Aimpoint H-1 or a small Trijicon RMR would be nice.
Keep your eye on the Optics section on the EE, most of the above come up for sale from time to time.