So building an AR-15 is hella expensive...

[Lexthedestroyer]

This I couldnt tell you.
But if I was to speculate I'd find that to be a grave waste of everyone's time as barrel length on AR rifle doesnt change its classification.The shortest AR barrel I've heard of was 7" (and if memory serves me right it was a blowback system).

7.5" is common, yes, but there is also this.. https://www.outdoorhub.com/news/2015/07/28/video-is-this-the-shortest-ar-15-rifle-ever-made/