Castle Nut?

No. The buffer tube doesnt have to lock in any particular position so it doesnt need the receiver end plate or castle nut.

On a collapsible buttstock and compatible receiver extension (buffer tube). The castle nut and end plate index the receiver extension to the correct position. This isn't necessary on the rifle buttstock because the stock doesn't index off the receiver extension, but off of the lower receiver itself.

I'll second what yugo and Mr Clean said, with the caveat that it generally applies to A1 and A2 stylr stocks. Magpul has a few fixed stock options, and at least one of those requires a castle nut.

This Magpul fixed stock DOES require a castle nut.


This one does not


Neither does this one:


If you provide your model number, we can help you better. Until then, best answer we can give is... Maybe.

I put a magpul fixed stock on my A3 just slid of the telescoping one and slid the fixed over the tube and tightened it up works great, I did use the spacer it came with and make sure you get the correct one their is 2 choices commercial and mil spec tube.

Yes, you'll install the buffer tube indexing it over the buffer retaining pin (if it has an indexing cut out, a non carbine style tube probably won't have the notch).

Then you'll need to install the rear take down detent pin and spring, the spring will be hanging out some, so keep it flat or pointed barrel down. Then slip the stock over the tube and screw it into place. The stock acts in place of a separate end plate and retains the take down detent spring.