Here's my recreation of the HEL E4A. It's preliminary, of course, since some details will change depending on the processes chosen to make the parts.
I've even come up with a not-suppressor version for a not-NFA clone.
The not-NFA would have to be welded closed, I think. But it'd be easy to make the body in a way that the baffles could be dumped and the plain tube substituted.
The threaded part that goes on the muzzle (almost dead center) on the silencer version has vents to allow gas into the chamber that overlaps the barrel. I am not sure how many and what size those holes are. They'd have to be absent in the not-NFA version.