That's a hell of an awkward way to reload an underslung shotgun. Might as well use the Masterkey shotgun instead.
It's done like that because RMB-93 (along with it civilian variant RMO-93 "Rys'" ("Lynx" on russian)) have magazine on top, barrel at the bottom and to reload it you opening port cover at the top of gun -in underslang configuration this cover hidden under rifle's handguard.
WHy no masterkey? First, in 1993, when it were first showed, there weren't any other pump shotguns in Russian. 2nd, notice weight - RMB-93 is a very light gun, since it have no bolt, so this 9A-91+RMB-93 combo weight only 3,8 kg unloaded. And 3rd - both guns produced by one manufacturer)