I've read (mostly old threads on the internet) around people having issues with the X300U-B (thumb screw model) on polymer pistols from overtightening.  Seems there is a lot of comments that the X300U-A version (quick detach rail lock) is best for Glocks.  I'm curious if anyone here has first hand experience with the 2 models on the Glocks and which they prefer.  If the X300U-A is tight on the Glock's, I'm perfectly fine with running that.  However, nothing would bother me more than the light being "loose" on the pistol.

Any thoughts/recommendations?

@duckhook02

Get the B version.  Hand tight or use a quarter to tighten 1/8 past hand tight.  Never had a problem with the B version.  You don't need to gorilla it on.  Now, with the A version, I've had them loosen on me (with the correct keys) and I too don't like loose fit.  Hard to snug up (it can be done, but B is the better mounting solution).  The A version is two plastic rails secured by three small screws per rail (has been know to break off from impact).  The B version is aluminum rails.    

I bought an X300U-A for my 19x, swapped the U mount for the P and lock up is solid. Zero movement. ArmsUnlimited had a great price on the black and tan X300A-U

Have both. Pick which mechanism you like best. Haven't had an issue with either. I hand tighten the B and it has never worked loose on me ( on Glocks). If it's something you're switching often between pistols or rifles, the A makes more sense imo.

No preference, but usually find more of the -A model on classifieds for a good price.

I prefer the A on a Glock.  I’ve seen two Bs break the screw.  One on a G17, one on a Beretta.

Either of them work equally well. I'd recommend considering a TLR-1 HL as well. Its cheaper, shorter and has great spill/throw.

A models work and work well.

Better holster options with A

I prefer the B.

But the TLR 7A is perfect.

The Surefire X300U-A model is perfect and secure on polymer framed handguns like Glocks.  You even get attachment options in the package if you want to try another method.

And while its obviously bigger, I prefer the X300U over my Streamlight TLR-7A.

Have both. The clip on is for when you want to move it from gun to gun, I use the screw when I want it to stay on the gun under harsh conditions.

Here’s a question regarding A/B models. I want to grab a GCode OSL RTI mount holster for my 19x. They only list the X300B models compatible. Aren’t they nearly identical except for the mount?

Has anyone in the history of mankind ever had an X300U just fall off? I've beat on the A models in ways I'd be ashamed to admit and they've always held true. Can't say that for things with screws that aren't Loctited.

I have had other clip-ons like the Insight come loose/off. Hence my lesson learned.

I have seen many Bs fall off on the firing line. Never seen a A fly off.

Quite the opposite from here.  Have seen an "A" break off when an officer practiced the down officer shooting (on back between his knees) and accidently smacked his gun down on the light rolling to get in the kneeling position of fire.  Rail broke right off.  Another officer used his gun as a make shift impact device by striking a suspect on the side of the head with said version "A".  Same result.  

Might I suggest tightening down and checking said tightness (witness marks) when you chamber a round and holster your weapon before going to work?  Or are these the same folks that don't make it to the range often and fail during quals?  Common sense....oh wait, it's not so common these days.  

As far as the screw breaking off on the "B".  Never heard of that.  What are they tightening down to?  An AR rail?  They must be using a screwdriver.  You don't need to gorilla it down or just hand tight.  Check the previous posts on the 1/8 turn past hand tight.  

I've had Laser Products, now known as Surefire since their inception.  

I have earlier generation X300s with "A" mounts and even the X200A and X200B with the "A" mount.  They were okay, meaning you can easily remove, but they have a tendency to loosen/wiggle/wobble.  The original X200 series had two screws per rail.  They had issues with a lack of rigidity and went to three screws per rail.  Then came the "B" mount in all aluminum.  My "A" versions are relegated to home/safe guns with the "B" versions for duty/carry.  I have a lot of the X300U B lights.  That's what I learned from carrying for several decades.  OP you'll have to make your own decision and see what plays out.      

A friend had one fly off his double stack 1911 when doing a mag dump. He didn't install it correctly and the lock bar wasn't fully latched in.