Pachmayr Gun Works 1081029: This ultra-rigid lightweight aluminum unit straddles both sides of the receiver and is crafted with a non-glare anodized finish. The top Picatinny rail can mount a variety of accessories, yet is designed to sit just high enough to allow continued use of the shotgun\'s bead sight. This rail mount incorporates a Slimline SideSaddle shotshell carrier made of flexible synthetic rubber to keep six (6) extra shells in readiness. This model fits 12 gauge Mossberg 500 and 590 models.