I was looking at lights and found this on another group who had the exact same question. why not use the X300 on a rifle. well this was thoroughly beaten down by guys who are mil/leo. you know the guys whose lives depend on gear. anyway here is an excellent response. I am not this guy
A few comments on using Surefire's Xseries lights on a rifle/carbine-
Most like the idea, and the reasons are numerous. It's bright, a hair lighter than the M 600, takes up less space, is mounted very close to the rail and are the least expensive "weapon rated" lights by Surefire. For most, these reasons alone are very attractive and from me you won't get an argument.
However, it's not all beer and skittles...
The weakest parts of the set up are how it mounts to the rail, the mounting plat and the end plate.
The light was designed for a pistol, these weaknesses are relatively transparent when used in that capasity. Here, the light sits snuggly in the 90 degree angle made by the trigger guard and the dust cover. If you were to have an impact related incident with the bezel/head portion of the light, that impact gets soaked up by the rear plate against the trigger guard, and not the mounting plate and rails.
However, on a R/C, the rail and mounting plate (both made of plastic) take all the brunt of the force. Also on a R/C, the end plate/switch stands alone and un-afraid. Not only is the trigger guard not there to provide a level of protection for it, but it's also the easiest switch to bump and have a light AD.
In my time at SF, I had 3 different organizations that loved the idea of running X300s (at the time) on a long gun, but had issues doing so. This was the real reason why we pushed M series lights for long guns. Sure, to the unknowing, it may have seemed like we pushed the M600C because "it's more expensive." Sorry guys, that's not true.
NOW, with the X300 U boat, I too am scratching my head as the juice may be worth the squeeze. But in the end, I think the Fury , in a proven mount, may be a better choice for an R/C if 500 lumens is wanted or required.
I'll more than likely continue to use my X series lights on pistols...and I'll wait patiently for the 500 lumen head for the Scout.