I came across a picture I liked.  This was it.

This prompted Steven den Beste to ask the following.

“There’s something down there next to the AR-15 that looks like submachine gun. What is it?”

Can’t say for sure because I don’t own the guns myself, but it looks to me like a Short Barreled Rifle.  (SBR)

(Picture source.)

The ArmaLite firearm company has always been interested in standing out, making something new.  The AR-15, which went on to be adopted by the US military as the M-16, is kinda-sorta modular.  What I mean by that is you really can’t take one rifle and simply swap out a few parts to make a completely different gun with distinct handling characteristics and capabilities, but the basic design does lend itself to customization pretty well.

Civilian firearm enthusiasts in the United States are always looking to push the envelope, Lord love ’em!  Someone got the bright idea of acquiring the shortest barrel possible, with the teensiest of shoulder stocks, and making a really compact rifle.

Okay, so you get a really small rifle.  What is it with this SBR stuff, anyway?

Well, if the gun has too short a barrel, or if the overall length of the weapon is too compact, then the firearm has to be registered with the US government.  A special fee of $200 USD (£150) has to be paid before taking possession of the gun.

All of this applies to guns which were built with shoulder stocks.  What happens if you build a cut down AR from scratch that never felt the taint of a shoulder stock?

Oh, that is just a pistol.  You know, a handgun.  Special fees and registration with the US government is not necessary.

Be careful, though!  I’m just belting out a brief outline of the law, and you could get in real trouble if you miss a detail or fail to follow a fine point.  If anyone reading this wants some of this nifty hardware, then I strongly suggest that they hire a lawyer who specializes in Federal and local firearms law in order to keep out of trouble.  You can get a little more in depth info by going here or here, but those two links are no substitute for professional advice.  Best to hire the lawyer and make sure all bases are covered.