When you look at the weapon used in the Dayton, Ohio shooting spree, it would be easy to conclude he used an AR-15 style rifle. But because the barrel on the AM-15 gun is so short, under ATF rules it qualifies as a pistol. Ammoland explains:
Pistols must be designed to be fired with one hand, without a stock. Firearms that have a stock and a rifled barrel less than 16 inches in length are called short-barreled rifles (SBRs). Shotguns that have a stock and a barrel less than 18 inches in length are called short-barreled shotguns (SBSs).
If a firearm is designed to be fired with one hand, it can have a pistol brace attached to aid in firing it with one hand. The AM-15 appears to have a pistol brace attached to the buffer tube of the pistol. The barrel is too short to be considered a rifle. Because there is no stock, the firearm is a pistol, not an SBR.
This illustrates how ridiculous the National Firearms Act of 1932 and its regulations have become.
There isn’t any logical reason to legally differentiate between a pistol over 26 inches long and a rifle or shotgun or any other firearm over 26 inches long.
The 26-inch figure is a rough measure, a dividing line between concealable and less concealable firearms.
The divide exists because long guns are seldom used in a crime, while handguns are routinely used in crime. The push for citizen disarmament, in the 1960s, was to register and ban handguns. The citizens were repeatedly told that no one was interested in registering and banning rifles or shotguns.