My thinking has always been this: the receiver (or frame) should be defined as the part that takes the feeding device. That definition works for handguns, ARs, for AKs, HK roller lock guns, Ruger 22s but the ATF doesn't do it that way. Even for revolvers the cylinder is the feeding device so it works there. Right now we have definitions that don't work for example an AR lower is not a firearm per rule.