There are certain supplies you need for mounting a rifle scope, and Loctite or an equivalent product is usually one of them. However, if you find yourself short without a thread locking sealant, this tip should suffice in the short term.
While mounting a scope on my rifle I ran out of locktight so I made my own. All you need is a bit of monofilimint fishing line. Cut a small piece of line and stand it up in the hole before dropping in the screw. Tighten down the screw and you will have an instant solid fit. I have also used this trick on bows while mounting stabilizers.
Using fishing line this way will work for many more applications than just scope mounting when you don’t have access to thread lock.