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Just a note on the G2X/6PX suggestions... Im not sure the "Tactical" versions of Surefire's 6PX and G2X lights are safe for weapon-mounted applications. I know the "Pro" versions are NOT. Considering that even the G2X is still $100, I see no reason to opt for one of these over a standard G2LED or 6PLED. Their output is sufficient for defensive purposes.
I have a G2X Tactical on a VLTOR offset mount, awesome setup IMO.
If youre looking for something similar to the M300A at a little lower cost, you can look at the VTAC mini L4. doesnt come with the pressure swich, or socketed tailcap, but if you use it in a location where those are not necessary, it can be a good cost effective compromise to the M300 scout.