Rifles and SMGs especially are criminally under-loved, compared to pistols.
Also, a back holster or transportation bag is just kinda...weird, and not very good for quick-response situations! My recommendation is that we create a weapon mod that can only be installed on rifles/smgs... the sling!
Now now, I know it sounds silly, but think about it. It could be a 10k-15k weapon modification that only SMGs, rifles, and shotguns support. It would allow you to wear your weapon, but still function in a @default-weapon capacity, so that if you're attacked, you get a message about snapping your slung rifle/smg into your hands. It would appear on your body like a normal holster, something like Slung across %N's chest is a %weapon.