Typically a semi automatic STANAG pattern magazine fed rifle designed around the 5.56 NATO (.223Rem) cartridge. Other calibers seem to fit as well as long as the rifle was originally intended to shoot 5.56x45mm.
I don't consider a Ruger 10/22 or even the Ruger Mini-14 to be black rifles no matter what stock someone puts one in, but an AR-15 22LR dedicated upper could be slipped into the category.
Just my opinion though, I'm sure others have a different one.



























