Well, I'm probably gonna step on a landmine here, but maybe you could at one point.
In 2010 the ATF caught a lot of flak for intercepting a small shipment of airsoft M4 rifles. Here is one of many stories about that:
https://www.everydaynodaysoff.com/2010/02/28/airsoft-gun-seizure-apparently-toys-can-be-real-guns/
So a lot of folks automatically laughed at the idea, but either way some of the folks who already had the airsoft gave it a try...and I can't see any of those results on the Internet anymore. Basically, it WAS just an airsoft, but it was an Airsoft that had at it's core a closer-than-80% lower according to some people at the time. Here is a link to the "new" lower:
https://www.evike.com/products/47163/
Yes, the lower would mate with an upper. One 'smith at the time said it was 10 minutes with a Dremel and 1 new hole drilled to make it into a usable AR lower. Others said it would be just as easy to make a lower from a block of aluminum. I don't know for sure, but it was an interesting story at the time. Either way, it was no more a "machine gun" than any other AR lower (well, except that it could fire Airsoft BBs full auto.)