Please explain a castle nut to me

[Stick'n'String]

I'm working on gathering parts for my AR build and I'm using the AR 15 parts checklist that a fellow TBHer posted from Brownells. What is the castle nut, and is there another name for it because the parts checklist doesn't have anything about a castle nut on the list

[Drycreek3189]

A castle nut has notches on the top. If you set it down with the notches up, it looks somewhat like a castle, hence the name.

[Manimal]

Learned me something

[WhiplashTX]

It's also called a lock ring on the AR15, it's usually included with the buffer tube.









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[hellbndr23]

http://www.midwayusa.com/product/718153/dpms-receiver-extension-buffer-tube-lock-ring-ar-15-lr-308-carbine?cm_vc=ProductFinding