Quoted:
airman100, did you figure out what the problem was? Could it have been too tight?
I guess by definition, it broke because I put too much tightening force on it
and I did that because it kept flipping off the P99 rail.
The rail of the P99, wasn't damaged, either- it looks like the rail on the P99 is undersized. My P99 measures 0.820 edge-to-edge. My 3rd gen Glocks measure 0.835 (four samples) and the rails on my ARs measure 0.830 - 0.0832 (YHM, Daniel Defense, Troy).
If you look at the vee shapes on the TLR-3 jaws, there's a long ramp on the fixed side and a simple vee on the moving side. I think the sweet spot for maximum contact is probably in the 0.830+ range (following the spec, what a concept!). With an undersized rail, it probably doesn't have enough contact surface to fight recoil, and the light will eventually flip off. Or, you can overtighten it, get to to stick, and have the rail fail two weeks later
So the moral of the story- don't put the TLR-3 on an undersized rail. Streamlight does state in their instructions this light should work on a P99, with a particular slot lock block.
I wonder if I went "Red Green" and put a single thickness of electrical tape on the rail under the mount on a new one it might stay on..