The Uts-15 in a pump action shotgun with a capacity of 15 shells. But that's isn't the only feature, it holds it's shells in to separate compartments 7 in each (+1 in the barrel) it can swap between these allowing the user to use two types of shells for example 8 buckshot and 7 slugs.
Implementation
The way I see it you could have multiple shell choices (Buckshot, slug, fragment, ect.) You would pick these at the attachments screen with two attachment categories the first for the main shell type, and the second for the alternate shell type or you could pick a laser sight and have 15 of one type.