That's a section of an HEL M4 noise suppressor. Mounted to an M16A1 it's way overgassed and needs an additional gas vent hole on the bolt carrier. That makes it an integral suppressor because without it, too much gas is vented and your M16A1 is a straight pull bolt action.
Take 5 baffles off the front and shorten it the length of those baffles and you have an HEL E4A noise suppressor. It's louder than the M4, but can use a standard bolt carrier.
I think that I can scale that drawing.
Time for another Form 1 it looks like!
Damn I have too many irons in the fire.