B-Square Mount Base for Remington 870 Express 3.5" MagB 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.