Weer'd Comment

His comment system seems to have barfed, or he's blocked me.  Sad face if the latter.

My comment to his post would have been:

Actually the measurements for a selective fire or even full-auto AR make a needlessly complicated plumbing fixtures SMG.

I've got the plans for a STEN around here someplace from when I was considering making a semi-auto clone.  95% of the bother with the design was ATF compliance and my intention of it looking the part.  If you don't care if your Home Depot SMG LOOKS like a STEN or has even a single part that interchanges you're in business for less than $100 and a day (assuming you have cartridges).
The HDSMG is not going to last long because they don't sell the quality steels that a "real" gun would need; but mild steel stock would certainly last long enough to be a full-auto Liberator.  Or a throw-away spring gun.

WW2 era submachine guns are brutally simple and cheap.  Even doing one up right like my on again-off again scheme is a lot cheaper than a "real" gun.  The most expensive part, hands-down bar-none is the barrel.  A rifled barrel is probably the hardest thing about a gun for someone sourcing the whole thing at Home Depot.

The thing is...  I've seen all the barrels you'd ever need for the project at a gun show where they'll take cash...  Replacement barrels for all manner of popular pistols are readily available, as are magazines in various capacities.  All you have to do is be willing to break the law.  Because making any SMG without licenses and approvals is breaking the law.