I have a viridian x5l gen 2 on my hk45c and I love it! I is compatible with the usp fs tac but you need a rail adapter. And if you do decide to go this route just a heads up that once you put the battery in and screw on the cap tighten the living sh*t out of it or it acts difficult otherwise it's amazing I rate it a 9.5/10 (-.5for the aforementioned problem)
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