These SureShell polymer shotshell carriers for the Mossberg 500 and 590 feature mounting brackets with integral top Picatinny sight rails. Using the top mounting screw holes found on Mossberg 500 and 590 shotguns made since 1997. The SureShell polymer carriers are thus slung on the left side of the receiver, securely fastened at the top. No other shell carrier mounting method is as secure or looks as good.