"It´s not that illogical!
The RG88 is made for live ammo, the shotgun is technically a muzzleloader"
No, that is still illogical.
The RG88Gum is a _NON_LETHAL self-defence device. (No, it is almost impossible to convert for metal-bullet ammo!!!). And it is in the same category as a .357 Magnum.
My Sharps percussion rifle (for example) in .54 caliber is a "freie waffe", but capable to kill a Buffalo from 100 meters.
I think the categorization of firearms should be based on their dangerousness on human life.
A 7,5 J 4 mm M20 gun is as harmless as an air rifle. A 35 J 10 mm rubber bullet shooter is even LESS dangerous than a 7,5 J air rifle. (Less penetration). Why do we (you and I) need WBK for them?