AR-10 .308 16" Black Hole Weaponry Mid-Length Stainless Steel 1:11 Poly Barrel. Black Hole Weaponry Barrels employ the use of 416R match grade stainless steel. BHW Barrels specialize in Polygonal rifling; The shape of the lands and grooves are based on a circle, so there are no sharp corners or edges for the bullet to conform into. This means there is a tight seal and thus less bullet deformation. This rifling form is unique in its ability to allow the bullet to be centered and fully supported by the barrel’s bore.This arc shaped rifling has long been held in very high regard for its accuracy, increased velocity, reduced gas cutting and ease of cleaning.