From CNN:

“That part of a firearm which provides housing for the hammer, bolt or breechblock, and firing mechanism, and which is usually threaded at its forward portion to receive the barrel.”
The problem — and this is where O’Kelly comes in — is that he says roughly 60% of the guns in America do not have a single part that falls under that definition. The AR-15, for example, has a split receiver — one upper and one lower. Neither meets the requirement on its own.

“For 50 years, ATF has been making this square peg fit in the round hole,” O’Kelly told CNN, “when, in fact, it doesn’t.”