Question: what is the difference between surefire x300u-a and x300u-b?
Answer: The only other difference between the two is the mounting system. The X300U-A uses the older style Rail-Lock® Mounting System. which is best suited for polymer framed handguns. The X300U-B uses the T-Slot Mounting Rail System which is best suited for metal framed handguns as well as AR-15 or similar style rails.