An AR15 Castle Nut, often simply called a "castle nut," is a small piece component of the AR15 rifle (and similar platforms), that threads onto the buffer tube and holds the receiver extension and lower receiver end plate in position.
The name likely comes from its close resemblance to the crenellated top of a medieval castle.
Here's a breakdown of its function and importance:
The primary purpose of the castle nut is to secure the buffer tube (or receiver extension) to the lower receiver of the AR15. The buffer tube houses the buffer and buffer spring, which play a role in the cycling of the rifle.
It is located at the rear of the lower receiver, where the buffer tube threads into the receiver. Once the buffer tube is screwed into place, the castle nut is threaded onto the buffer tube and tightened against the lower receiver to lock everything in place.
The notches on the castle nut are designed to be used with a specialized tool called an AR wrench or armorer's wrench. This tool fits into the notches, allowing for the tightening or loosening of the castle nut.
Right behind the castle nut, there's typically an end plate that sits between the lower receiver and the castle nut. This end plate often has a loop or other attachment points for a sling and also keeps the takedown pin detent and spring in place. When tightening the castle nut, it also secures the end plate in its proper position.
After the castle nut is properly tightened, it's common (though not always done) to "stake" it. Staking involves creating a small deformation in the metal of the castle nut and the end plate to prevent the nut from unintentionally backing out or rotating. This provides an extra level of security, ensuring that the buffer tube remains firmly attached to the lower receiver.