I second the Vortex Sparc. I'm using that on my UMP conversion and I'm digging it. It co-witnesses perfectly with the front sight (I guess the rear aperture sight, too, but I find it a nuisance to use) and the 3 MOA dot is small enough that it doesn't obstruct/obscure the sight picture when shooting for distance. Mine's sighted at 100 yards although I'll probably be re-sighting for 50 since it shoots about 3-4 inches low at pistol range distances.