Probably rifled bore and rifle sights. Or rifled bore and rear sights.

They are often made up for each product and can mean different things. Like "NS" sometimes means No Safety, other times it means Night Sights. Its just limited title space that they try to jam every identifying feature into. Sometimes its difficult to tell what they are trying to indicate. In other words, its rarely a universal acronym, its often Buds made up lingo.

RS = Rifle Sights and RB = Rifled Barrel (For Slugs).

RS – Rifle Scope