I remember firing a Mac-10 in .45 ACP once, shot so fast it ejected some unfired rounds which we loaded back up and sent down range! Fun stuff. M3 Grease Guns and Thompsons are more practical and controllable.
The Modified MAC's are actually winning some of the machine gun competitions. There are some
cool uppers you can put on MAC's to slow the rate down, better stocks, caliber conversions 45 to 9 mm, 22LR and even 5.56 is available too. I think someone once made a .556 belt fed upper for a MAC too but, ATF was not too happy with it.
I worked for a company that sold full auto weapons / NFA toys in Fort Pierce back in the 80's when they were pretty cheap. MAC-10's sold for a few hundred bucks and a M-16 for under a grand. Today a MAC-10 will sell for $8000 to $10,000 and a M-16A-1 for around $30,000 They were a good investment...