I know they consider it a firearm (at least for a bolt action). For some reason, on ARs it’s the lower receiver that requires an FFL, and not a barreled upper.
From a common sense perspective, a barreled bolt action is no more a firearm than an inert Claymore is an explosive. I know what regulations say, but common sense says something else. It’s no different than arguing that half of a disassembled desk is a desk for regulatory purposes. You can put that into law, but it sure doesn’t make it a desk.
Oh, I would never argue that federal laws make logical sense, especially concerning firearms.
The issue with AR lowers is hilarious, as all the lower really is, is a trigger housing and a magazine guide. An upper can be loaded and fired (disable the gas system) as a single shot, without the lower. But as you point out, the ATF has decided that the lower is “the gun”.