I think this video will help you understand the parts and how they fit together

one note is that barrels generally come with the extension already threaded on and head-spaced

and the BCG gas key generally comes attached and staked already also. most of the time you buy a BCG bolt carrier groups ready to drop in the rifle

how it works

the forward assist was not in the original design and a good argument can , was and is still made to not have it or not use it.

if your bolt does not return to battery it is generally an indication something is wrong , dirt , bad round ,

the air force ran the rifle without a forward assist for a few years before the army wanted in and decided it needed it , because you could smack the handle on the M14 and M1 they insisted the M16 have it.


if you use the forward assist it will may well be the last round you shoot without using a rod to get the old case out , or a broken case extraction tool. so make it count.

It takes a little to learn the lingo -- but don't let that stop you. It is simpler than you would believe. It can be as simple or complex as you want to make it. Kinda like trapping.

If someone asks what time it is here, everyone wants to tell them how to build a clock just to convince everyone else they know how.

You’ll be lucky to find one for sale now, if you find one buy it.

Sheesh, I thought folks were just sharing information and knowledge.
If you go to PSA, Midway, Brownells, and so forth you can easily find
the parts to build you one right quick. Not as cheap as it was and good
luck finding ammo. (at least reasonably priced ammo)

Zim

A search on Gun Broker for AR-15, brought up 1395. I think I could find one. Some of it is stripped lowers or complete lowers, but I would bet there is over 1000 listed.