I have a MX400 on quick release mounts so it goes on in a flash and like ranger56528 stated, I too wanted a light for outside use but what's so cool MX400 is that it's variable in lumens intensity. I can use it indoors or outdoors. It even has a blinding strobe setting that is insane in the house! But on the lowest setting it's perfect in the house.
The photos below show how close it fits to the rail. One photo shows it on but a background light was used as to get the photo and see the rifle, but the photo of it in the shooting tunnel is what killed me. That is the ONLY light on! The walls are white sound absorbing fire resistant board for 15yrds. The rest of the tunnel is dense black sound foam for the next 85yrds until the 100yrd big targets. And look how well it illuminated them at 100yds with no other light reflecting off of the walls from 16yrs on down to 100yds! That light really impresses me. I used it in Africa to shoot some spring hares at over 80 meters in the black of night at about 1am. The PH I was hunting with wouldn't let me get on the plane with it he wanted it so badly. Lol. One cool thing is that the push button tail switch can be swapped out for a wired Surefire tail switch of which I now have (if I want it set up that way.) It's so nice to be able to take it off and have it as the bedroom flashlight right next to the HK USP each night and then in 5 seconds throw it on the rifle if I want to.