[longtermmemmory]

ok forgive this basic question, but what is the deal with “assembling” a do it yourself ar?

and what exactly is a stripped receiver?

[Gaffer]

it is usually a cast forging for an AR. It is the bare serialized main component of the lower receiver (the part that is tracked). You have to install all the myriad parts on it to mate with an upper which isn’t tracked.

[Charles Martel]

The lower receiver is the serial-numbered part... the "firearm", according to BATFE. In this design, though, it only serves to hold the trigger, hammer, safety, magazine and recoil buffer in the correct places, when latched together with the upper receiver and barrel. You can buy the lower as a "stripped" but completed part - you'd have to buy the small parts (trigger, hammer, springs, pins, etc.) and assemble it. This requires a bit of attention and a set of pin punches, but it's quite easy.

The tough part of assembling any rifle is attaching the barrel and setting the chamber headspace. This is already done for you on the "completed upper assemblies"; you just have to mate the upper and lower halves. So the portion of the AR-15 that contains the explosive force of the cartridge is technically just a part or collection of parts; it does not have to be shipped to a FFL holder. Lower receivers (built or stripped) and completed rifles do have to be shipped to FFLs.

YouTube probably has lots of AR build videos. Here's another site that illustrates what I'm describing:

www.ar15builder.com