It's incredibly stable. And I only tightened it up with a quarter, have fired a couple of hundred rounds through it, lay it on and off the nightstand several times a day, dry fire it, go through reloading of magazines (banging the things in and out of the mag well) and have noticed no loosening since the first time I put it on. Yet, I could get it even tighter if I chose to use a large screwdriver to crank it even tighter. But I haven't felt any need to do so as yet...partly because I may remove the light if I decide to carry the weapon once more holsters become available.
Since it uses a 1913 rail slot it sits about 1/8 inch forward of the of the trigger guard. But my forefinger can still manipulate it, yet there's no danger of accidently turning it on if I didn't want to.
The battery compartment has a tension clamp that I had to pry open with a small screwdriver and then really push hard to close. It wasn't "easy" but not difficult. Definitely secure.
Thank Art. I was glad he posted that picture, too, as I've been going back and forth on one of those for awhile now. I'm just not sure how I feel about mounting anything to a pistol. I guess if I didn't like it on the FNP, I could get the mag tube adapter for my 1300.