M*CARBO Brotherhood

Adapt Castle Nut For Sling Mount?

I have a new Sub 2000. After looking for several years, I finally came across one at a terrific price and in stock, and am getting ready to nearly DOUBLE my cost by adding the whole nine yards of upgrades available through M*CARBO (except the muzzle break)! ;^)

I have seen a couple online situations where someone cut narrow slots in the castle nuts of the Sub 2K from the open end to allow a tightening band to cinched around the nut, helping to hold it in place. Has anyone here tried that with the M*CARBO single point sling mount? It seems like it could possibly help hold the castle nut in place and prevent it from spinning, yet make it easier to remove for later upgrades or deep cleaning.

Just thinking things through before I start my upgrade… and upgrade… and upgrade… and…


@Hawkeye I have not tried the cutting of the castle nut, but I use Loctite 380 Black Max and have not had any loosening of the nut.


Thanks! I have some of the 380 coming, but thought it worth asking about the other idea while I had everything torn apart.


I guess I got lucky on my Sub 2k’ Never Had issues on the Castle Nut Loosening Up! Yes the M*CARBO mount with the Proper tightness will help and should Insure a Tight Fit,Using the Sealant they Recommend and good cleaning of the Internal threads of the Nut Itself, I use a Dental Pick to help and Alcohol and for the contact surface on the grip. For the Hard Removal of the nut A good known (Flat Punch)Not a (p.o.s.) Rounded Off Thing in the bottom drawer of your tool box’ Tapping on the bottom of it in a couple of spots will assist on this When Having trouble with MC’s wrench. Hope this Helps.


I have the MCARBO tube cover and it butts right up to the castle nut. I used the LT380 and have the single point sling adapter on the firearm it hasn’t come loose yet.


Thanks, all!

What is was considering was cutting two slot is the sides of the castle nut with my Dremel to allow the sling mount to “pinch” the collar in place slightly. If it was a flop, a replacement castle nut only costs a few dollars.

I guess at this point I may wait and see if the nut gives me any challenges staying in place, then address the issue when/if it happens.

Thank you for the input.


Is there a real function for the castle nut? Especially since the threads don’t really secure to anything and you have to use loctite to keep it adhered? Would there be anything wrong with just leaving it off?


Make sure you store the leftover 380 stuff in the fridge, as per the instructions. I missed that the first time around and it was all dried up when I went to install the prototype trigger bar.

1 Like

Other than looks, I don’t really think it serves an actual purpose regarding function. The receiver halves are screwed together plenty tight without it. Plenty of people have shot their SUB2000s with loose collars, either knowingly or unknowingly, and I’ve never read a single time where that actually caused a malfunction.


Thank you Sir. That helps with some ideas I have for different mounts and using that space.