There is a blog specializing in documenting open bolt semi autos, which shows a lot of old ATF file notes and rulings on various open bolt actions. It might give you some of the background to what is halal and what is haram, with such guns.
While you are there, check out the stuff on the BSA "Ralock", it's certainly different.
I don't know anything about the current crop of MAC 10 clones, and whether they are open bolt, or closed bolt with striker or hammer firing, but I can think of several ways to make a gun which fires from an open bolt, but which would not be easy to modify to fire full auto. Whether they would:
- appease the BATFE
- Sell in sufficient numbers and at sufficient price to turn a profit
are completely different matters.
Some of the open bolt semi autos, like the Gevarm .22 rf range, were designed so that the recoiling bolt disconnected the sear from the trigger each time. Some people were caught with examples converted to full auto, but I'm guessing that the conversion would not be straight forward, especially to selective fire.