If I want to build an XM177E2 upper, how do I figure out if the grenade ring behind the flash suppressor (moderator) is legal or not?

Seems I recall the grenade ring on a French MAS 49 is okay, but most other rifles are a no no.

To me the grenade ring is innocuous because it cannot be used for anything (like that ever stopped gun banners from making stupid laws that have no effect on anything.)
So what laws prevent the use of the ring, and how can I tell I'm legal in using it or not?
Supposedly the first XM177E2'd didn't come with ring, but I can't confirm that either. But that will be the way I have to build it unless someone can prove otherwise.
Help appreciated.

Seems I recall the grenade ring on a French MAS 49 is okay, but most other rifles are a no no.

To me the grenade ring is innocuous because it cannot be used for anything (like that ever stopped gun banners from making stupid laws that have no effect on anything.)
So what laws prevent the use of the ring, and how can I tell I'm legal in using it or not?
Supposedly the first XM177E2'd didn't come with ring, but I can't confirm that either. But that will be the way I have to build it unless someone can prove otherwise.

Help appreciated.

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