B-Square Mount Base. For use with Remington 870 Express 3.5" Magnum, Remington 870 912 & 20 gauge. The B-Square sporting Rifle Mounts require no gunsmithing and install in minutes. Each style of mounting rails allow for maximum scope placement options. Made from 6061-T6 aluminum. these mounts are durable, yet do not add unnecessary weight. B-Square mounts are the easiest way to add a scope to your rifle..