While it's not the best light option it is very cheap and effective (at least I've had no problems yet). I use a streamlight polytac mounted on an elzetta 1500 fsb mount.
The polytac is not an upgradeable flashlight, but does come out of the box with a c4 led, uses cr123 batteries, and has a 3 hour runtime with an output of 120-130 lumens. The major caveat for the polytac is the size of the body, it is not a 1 inch diameter (I think 0.8 inch diameter) so a layer or two of the stretchy electrical tape is a good idea for a secure mount (on most flashlight mounts).
The elzetta 1500 is awesome (for me). the sport is a carbine gas system on a 16 inch barrel so there is a lot of barrel hanging out of the handguard. The elzetta mount allows the flashlight to be mounted far out front (further than the typical offset or scout mounts). I don't see an issue with barrel shadow, it is there but not obstructive (only if you had your barrel buried in someones chest the shadow would block their face) other than that at room distances your irons will be close to the top of the focused beam (barrel shadow a few feet above that!
Not the BEST, but a cheap and effective solution for a flashlight and mount (about 50-60 for light and mount) especially if your using plastic handguards!