I just got an arms 40L BUIS to mount behind my ACOG.  It came with Loctite.  How necessary is the Loctite?  I am asking because I will only be shooting at the range, and MAY occasionally switch back and forth with the carry handle.  Thanks in advance.

Well, if the people that made it provide loctite then I believe that should tell you more than the info you get here.  I BLUE Loctite just about every accessory hanging off my AR, except things that quickly release.  I BLUE loctite my BUIS, my KAC sling swivel mounts, etc.  I use a little loctite on my M4 stock castlenuts.  BLUE loctite will break with common screwdriver torque (USE THE CORRECT SIZE GUNSMITH QUALITY SCREWDRIVER) if you don't overuse it.  A drop or two is all you need.  As for a BUIS, if I could only Loctite ONE THING on an AR, that would be it.  Red loctite is only to be used on things you do not want to take off.  DO NOT USE RED on your BUIS. Blue prevents vibration from loosening the screw, but allows you to remove it.
USE BLUE

Quoted: I just got an arms 40L BUIS to mount behind my ACOG.  It came with Loctite.  How necessary is the Loctite?  I am asking because I will only be shooting at the range, and MAY occasionally switch back and forth with the carry handle.  Thanks in advance.

Thanks for the informative response Airbiscuit.  I appreciate it.  I will go ahead and use the Loctite provided (kind of purple looking, but is probably the blue).  Knowing that the seal will break with a strong twist of a screwdriver makes me feel better about it.  Thanks again.

be sure to shake it up!

like as stated above.  i use blue on all threads not designed for quick release.

My pleasure.  Just remember, from a loctite fiend (me): 1) Don't overdo it....a drop on the threads is more than enough.  Really don't overdo it if you use the stuff on a M4 stock castlenut or you will be cussing and may damage or scatch something  2)  Use the correct sized screwdriver.  Get a quality ground gunsmith screwdriver set (easily less than $50 with interchangeable heads).  

Quoted: Thanks for the informative response Airbiscuit.  I appreciate it.  I will go ahead and use the Loctite provided (kind of purple looking, but is probably the blue).  Knowing that the seal will break with a strong twist of a screwdriver makes me feel better about it.  Thanks again.

Indeed, what hk said.  sorry for not mentioning that.

Quoted: be sure to shake it up! like as stated above.  i use blue on all threads not designed for quick release.

Thanks HK & A.B.!

First time I shot using my #40 I did ok, got it sighted in, then set it aside for a few minutes while I worked on another rifle.  Came back to the flattop and my shots were all over the place.  Seems that the BUIS had worked loose during firing (maybe 60 rounds).  

Now I locktite it.

I locktite.

Quoted:  I will go ahead and use the Loctite provided (kind of purple looking, but is probably the blue).

 That's cause it probably is purple.  The purple loc-tite is even easier to break than blue.  

Quoted: I locktite.

+1

Cheap insurance...

The purple, I believe, is for threaded screws over 3/4".  The blue loctite has a weird blue hue to it sometimes, that may appear purple.  If you are wondering whether it is purple or blue, it is probably blue.

Quoted: Quoted:  I will go ahead and use the Loctite provided (kind of purple looking, but is probably the blue).    That's cause it probably is purple.  The purple loc-tite is even easier to break than blue.

I lost all the mounting hardware on my #40L. $10 from ARMS for new hardware and loctite.

so yeah, loctite

If it has screws and goes on an AR15, there should be blue loctite on the screws.