Heads up! Good article with a lot of unanswered questions.
I recently saw a Mac-10 clone chambered in 5.7x28.
It uses the FNC pistol magazine that holds 20 rounds, expandable to
30 with a conversion kit. (10 rounds for you poor saps in CA.)
I considered getting it until I found out the store had been out of
ammo for two months. Apparently someone is taking all the the stuff produced and there is precious little left over. It is a fairly obscure cartridge and there are only a couple of production sources.
Also, it operates at pretty high pressure and the cases have a special coating to make them feed properly so reloading is tricky.