Used to tighten the carbine buffer tube to the lower receiver, the US Military has used two different versions.
- Lock Ring- Original Style, used on XM177's, early Carbines, etc. Long out of production so this is a very difficult item to find. Needs the Lock Ring Tool for tightening (Here)
- Castle Nut- Replaced the model above and is still the current style. Typical for later model Commando's and M4's. Many AR tools have a solution for tightening this castle nut.. there is a special tool for this if you need (here)