Billy, the three .19-Cal hoons (isn't that a funny way to say that?) are different from the 5mm - which are .20-cals.
Not much difference there, I agree, but different. The .20s aren't hoons.
Catfish is correct. The modification done to those 5mm Rimfire Magnum rifles is simply a way to keep the rifles in action. Since 5mm ammo is about gone and terrifically expensive, the only way to keep the guns from being wasted is to convert them to a nice mild centerfire. The .20 Hornet is just about perfect for that application.
The .19 Badger - based on the .30 Carbine - is tempting, I agree.