have you tried either?
I was really unimpressed by the quality of the M500xx
the rocker switch feels really clunky
as far as the beam...you got it all wrong
the M500AB and M500B will have a tighter beam than the M500A, the larger reflector focuses the light to a tighter beam, none of the them have adjustable focus
M500A is the same as a SF M3
the M500AB is the same as a SF M3T
the M500B is the same as a SF M6
personally, if you have quad rails on your carbine...i'd go with m900 or m910
i went with the M900A, it uses the smaller head, which has a wider beam than the big head....now by no means is it just a all over the place wide beam...it is still pretty well focused.....if i ever need to use the light ...it will be indoors...were a wider beam would be more usefull than the larger head , also the m900 has navigation lights....so you have 2 small leds...to help you navigate without killing your night adapted vision
it all depends on how you hold and shoot your rifle
i prefer either vertical grip or magwell hold
if you like holding by the handguard, and your a righty...it will work good for you