I tried the FSB mount for a while but abandoned it (same mount you're looking at too). Ultimately I decided I wanted a longer forearm and to mount the light farther forward (which also helped to reduce/eliminate barrel shadowing from the light). When bolting a light to the FSB, you are essentially adding weight/harmonic variance to the barrel so your zero/impact may change. Granted, the impact change probably won't make much of a difference at any distance where you're actually running the light, but in the long run, mounting the light to the rail seemed to be more secure and resulted in fewer variables for me. That said, that GG&G mount excellent quality and is GTG... just make sure you loctite the mount.