[Professional Engineer]

I’ve not heard of this in a rifle barrel before.

[davetex]

This is the manufacturer.

http://www.blackholeweaponry.com/

The barrel is black melonite Stainless Steel.

[The KG9 Kid]

It’s more of a Euro thing, and usually German. Hk produced rifles with polygonal barrels in the ‘70s and ‘80s.

[davetex]

I think I remember LaRue also uses some polygonal barrels on a few rifles.