Introduction
Many firearms have problems with screws coming loose, especially magnum rifles and handguns.  Marlin rifles are notorious for having screws come loose, especially under heavy use like Cowboy Action Shooting.  The Internet is full of stories of screws coming loose after only a couple hundred rounds. 

The above photo shows the Loctite® products I use most.  242, commonly known as Loctite® Blue, I use on screws.  It prevents the screws from coming loose, but the screws can be removed with common hand tools. 

I almost never use 271, commonly known as Loctite® Red, because it is permanent.  I have used it on screws that I know I will never remove, or don’t want to remove, such as a sling swivel stud screwed into a metal front barrel band on a rifle.  271 requires heat to break the bond and remove the screw.  242 and 271 can both be purchased at hardware and automotive stores.

Loctite® 609 is green in color and is used in press-fit applications such as bearings.  You can find it at specialty stores online.  I use it on the base and dovetails of sights if the sight is loose in the dovetail.  The bond is semi-permanent; I use a punch to break the bond to remove the sight.

A small drop of Loctite® is all that is required for most applications.  I have had the above Loctite® products for years; I just don’t need to use very much.  “A little dab’ll do ya!” as the old commercial said.

To use Loctite® Blue on screws, first I clean the thread on the screw and screw hole with acetone and a cotton swab.  This removes any grease or oil to make a good bond.  You would be surprised how black the swab becomes after cleaning.  For screws that have no shank, only threads like the trigger guard plate screw above, I apply a drop of Loctite® Blue directly to the threads.  If I get too much on the threads, I touch the drop with the tip of a cotton swab to remove the excess, then install and tighten the screw.  I remove any excess Loctite® that squeezes out of the hole if I can get to it.

For screws that have a shank, like the hammer screw above, I clean the threads on the screw and screw hole with acetone as before.  I put a drop of oil on the shank and remove the excess.  The hammer pivots on the shank so it needs to be oiled.  I put a drop of Loctite® Blue on a toothpick and apply that drop to the threaded hole, NOT the screw.  I don’t want any Loctite® to get on the shaft or rotating parts; it could cause the part to bind or not work at all.  After tightening the screw, I remove any excess Loctite® that squeezes out of the hole.

I have discovered that Loctite® thread locker products remove cold bluing, so I try to avoid having excess squeeze out of the hole on cold blue surfaces.  If this happens, I make sure to immediately remove the excess.

When installing scope mounts on receivers I first install the mounts to ensure the screws are not too long; I don’t want them to touch and jam the bolt.  I clean the screws and screw holes with acetone and use Loctite® Blue to install the screws.  If I can remove the bolt, I wipe away any excess Loctite® to prevent the bolt from sticking.  If I can’t remove the bolt, I am careful not to apply too much Loctite.  I once applied too much Loctite® to scope mount screws and it bound up the bolt on a .22 semi-auto rifle.  Once I was able to disassemble the rifle, I used acetone and a brush to remove the excess Loctite® from the bolt and receiver.  You would think a person would be smart enough to think this through and not apply too much Loctite, but when you’re young and inexperienced this becomes a hard-learned lesson!