an AR lower is considered a "firearm"....if you bought an upper he could ship it to you, but not a lower...to ship the lower is the same as shipping a whole firearm (same goes for the frame of a handgun...you can send the slide, barrel, whatever...but not the frame) (FFL to FFL is the proper way to ship a firearm to another individual)....the only time you could ship a lower yourself is if you bought it new and were mailing it back in for warranty work and sending it to the manufacturer...and im not sure if they could send it back to you or if it would have to go to an FFL for you to pick up also - the 3/16 he is talking about for an SBR is NFA stuff, its deep because its meant to be permanent, no etching it away lol