My screw assembly at the front of the lower receiver that keeps the upper locked down is fitted perfectly into the receivers but the screw that secures it comes undone a little bit too easy for my liking. Looking for suggestions. I was thinking Blue Loctite. The screw itself looks like it had red loctite on it once before. Thanks.

My screw assembly (read my colt front take down pin, screw style)

If you use loctite on to bond the retaining screw to the pin section together, you going to have a hell of a time getting them separated when the time comes that you have to pull the upper off the lower.

A better solution would be to use Blue locktite on just the inner threads of the screw, let if dry, then insert and tighten the two together.  This built up/ dry loctite will help to keep the two mated, yet still allow you to pull the screw when the time comes (read be able to back the screw out without the use of heat to break the loctite bond).

I have an Sp1 Carbine with the front screw take down pin.  My dont have loctite on them and they stay together great.  One is a screw and the other is what it screws into.  So long as I tighten them enough, they never come apart.  They move together as the upper comes off or you can twist one and they both turn.  

I hope what you were describing is rare causing I would rather not have to loctite something that really shouldn't need it.