I'm currently running a CA m15a4 (which is now turned m16 rather than the carbine) and i'm really happy with it. the gun shoots awefully fast so you'll need to invest in better mags than the hicap, it doesnt really catch up all that well with the rate of fire.
As for the rails? There isnt any size difference that I can see. everything i've mounted on mine fits snug.
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Extreme Vaporizer