[Charles Martel]

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.

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.

[ctdonath2]

It has to be something in unquestionably common modern US military use. The Army orders an awful lot of M4s, as distinct from M16s.