M4s - real, military-designated, select-fire M4s - were introduced after 1986. Therefore, either your friend’s M4 is an “M4gery” (close, but not quite), is owned by the military or police, or is illegal.
Many M16s have been refitted to be functionally identical, but aren’t what I’m referring to. It’s a nuance that is legally important.
You may be correct. It’s just another “mouse gun” as far as I can see. But since an M4 is basically a short-barreled M-16 why keeps him from making that conversion? The receiver is the NFA controlled part and since he had to get all sorts of permission to get that what difference can a shorter barrel make?
Oh, sorry. I forgot we were talking about the government here. Logic isn’t really welcome, is it?
Until the main firearm of the U.S. military is something other than an M-16 variant, you're talking about a distinction without a difference.
I understand the point you're trying to make, but you're using the wrong firearm as an example. Try a design that wasn't manufactured until after '86, such as the H&K P90.