I know of only a very few circumstances for thread locker to be used on a firearm - (IMHO of course). If a gun's threads and the screws are properly cut and the screw is properly tightened, it should not come lose by itself under normal firing. Properly is the key word here! Tightening fasteners is sort of a skill that is a learned over time and comes with experience. Too lose or too tight are equally as bad.
Yes, there are a very few exceptions. A Colt Single Action revolver is a design of over 150 years old. Those screws will absolutely come loose after a day's heavy shooting. Like they say, if you carry a Colt SAA, carry a screw driver -lol. Still, I do not use thread locker on mine. I just tighten them up after shooting and cleaning them so they are ready for the next outing. I suppose one can make the case to use a mild temporary thread locker on these revolvers.
The other times I could understand its use is for a trigger stop on a S&W revolver if it actually has one. On the rare occasion they come lose they could jam up the gun. If shooting a gun with a scope or optic - maybe there as well. If Locktite is used in these examples, I'd use either the green or blue only - never the red which is hard to remove and I see no need for that.
That said, I see no need to indiscriminately use Locktite on a gun if everything is in spec. and properly tightened. If I owned a S&W that had an issue, I'd check out both the male and female threads and replace the screw before using Locktite - but that is just me.
And..... if you absolutely do need to use Locktite, the screw and thread should first be cleaned with alcohol or similar to remove crud and oil. All that needs to be applied is a very small drop! You do not want any surplus coming off the threads and going into the gun which could cause even more additional problems.
Just my opinions of course - YMMV.
