[rickyrick]

Looks like they intend to, or can include AR uppers.

Under the proposed rule, a “frame or receiver” is any externally visible housing or holding structure for one or more fire control components. A “fire control component” is one necessary for the firearm to initiate, complete, or continue the firing sequence, including, but not limited to, any of the following: hammer, bolt, bolt carrier, breechblock, cylinder, trigger mechanism, firing pin, striker, or slide rails.

The proposal goes on to say...

More than one externally visible part may house or hold a fire control component on a particular firearm, such as with a split or modular frame or receiver.