I think you're hitting this nail on the head. (One of the things we have to remember is that we're subject to a bit of consensus bias in that we all are deeply involved in the firearms conversation, so we assume that the majority of gun owners are generally like us. In reality, most own one or two firearms and think of them as just a thing they own, rather than a part of who they are.)
Throughout the 90s and early 2000s, the estimate was 300-350M firearms in circulation. Based on
NICS checks, there are an estimated 350M+ transfers over the last 20 years, though everyone keeps that 350M pretty stationary. Obviously, some of those NICS transfers represent used sales, but it can't be more than 50% or there wouldn't be the healthy industry for new firearms that we have. That means 4-700M firearms in circulation, conservatively, based on NICS and the like.
With the explosion of 80% builds, it's probably not unreasonable to say that we're closer to that high end (possibly nearing 1B total) of which maybe 10% (SWAG) could be home-built, and the majority of
those might be ARs. I'd could imagine 100M ARs in circulation without a ton of effort, but certainly not 400M.