This is a big business, Umarex, Rohm, Record, ME and some well known real gun manufacturer (Taurus, Steyr, Grand Power, Alfa, Ceska Zbrojovka, Izhmash) manufactures such guns.
Just for informational purposes: Are those guns based on the sold-in-Germany Gas Models or are they a different kind of fish? We keep talking about purpose-made misalignment of barrel axis and chamber, incorrect barrel timing on Revolvers etc regarding the Gas Alarm Guns. All this would cause Misfunction in a Rubber Bullet Gun, I suppose?