I think I remember a while ago reading a statement by streamlight or surefire warning against possible malfunctions with their lights mounted to the frames of 40 cal Glocks... never heard anything more, but I'm curious if there's any validity to this. I'd like to mount one to my Sig P320, but am not sure I want to risk causing malfunctions. Anybody heard of this happening?

-Pops

That was a unique issue to the Gen 3 .40 cal Glocks, especially the 23's.

Caused malfunctions of the weapon and was corrected in the Gen 4 models with the new recoil spring setup.

Never been an issue in any other brands though to my knowledge.

Funny thing my G32.3 never gave me an issue with a TLR1 in 40 or 357.  It only effect some not all you either got a good or bad one.

Never had an issue with my Gen3 G22 with a Surefire X300u.  Running the same light on my 320c and after approximately 1500 rounds I haven't had any issues.

I experienced the problem on my G22 with a TLR1. From what I read, those that tightened the screw down too hard had this issue because it was somehow affecting the natural flex of the frame during cycling. The fix was just to loosen the screw, which I did and never had a problem since.

Whenever I had to remove the light and then put it back on, I would tighten it by hand til it stopped, then I'd loosen it by 1 or 1.5 revolutions and leave it there. Never had any other problems after I started doing that.

I believe the fix was an upgrade to the magazine. 

There were a couple of fixes including a new RSA and follower design. However as stated above it really came down to luck of the draw if you have a good one or not. I love Glocks but the way they are made there can be a problem with tolerance stacking. Guns that test well have problems once they heat up and the tolerances shift or the gun wears and the tolerances change. It is how you have a great gun that three hundred rounds later starts having problems. In this case I believe the addition of a weapon mounted light on the G23 frame was causing a tolerance shift. The Gen 4 guns got a new the new dual captured RSA as an upgrade.

Probably was the people who got the screw driver out and tightened it down.  I believe even Streamlight recommends finger tight plus it has the retention spring on the bolt rail lock up system.

No issues with lights on anything but the .357 SIG M&P.  I "lost" three batteries in my Streamlight in a couple years.  

When I'd try to turn the light on it wouldn't work, I'd pull the batteries and give them a shake.  The one that clicked was the "bad" battery.  I assumed that recoil was causing some batteries to fail internally.  I'd toss that battery, grab another one and install with the one that didn't rattle and the light would work the batteries went dead - or another battery failed.

No issues on 9MM or .40 S&W plastic framed pistols.

My Glock 22 Gen 3 was not reliable with a TLR-1.  It didn't matter how tight or loose it was mounted.  I fired thousands of rounds without the light and had zero malfunctions.  After putting the light on I fired 100 rounds and had two malfunctions.  The light came off after that.

Yep- My G23 ran flawless with a TLR1 attached to it