I've mounted a Streamlight Protac light on my M4 (using a side mounted pic rail attachment I already had from S&J Hardware and not the one Streamlight provides so I'm not rating that) and the fitment is just fine for the light and pressure pad at the top of the handguard. For reasons, I don't want a semi-permanent mount right now so I used industrial velcro rather than the excellent 3M tape provided. I have the 1000 lumen version Protac with two CR123 lithiums that really lights up the night, it's good compromise between a spot and a flood as it has both. Only thing is I don't know for how long it'll stand up to 12 G recoil and only time will tell that; however the hundreds of dollars of savings over a Surefire setup is just too attractive.
Edit: there's no lockout for the light so just loosen the tail cap when that seems advisable.
