"In some of the new letters, ATF has begun listing the following “objective design features” when making its evaluations:

Incorporation of rifle sights;
Utilization of "rifle caliber ammunition" (both 5.56mm and 7.62mm have been considered as such);
Incorporation of “rifle-length barrel;”1
The “weapon’s heavy weight;”
Ability to accept magazines that range in capacity from 20 rounds to 100 rounds, “which will contribute to the overall weight of the firearm”; and
Overall length of the weapon which “creates a front-heavy imbalance when held in one hand.”

... Under ATF’s new interpretation of the handgun definition, millions of AR-15 style pistols could be considered “too large, or too heavy” to fall within ATF’s new interpretation, thereby making them unregistered NFA weapons, and subjecting manufacturers and gun owners to criminal prosecution."