There was also the "Ljutic Recoiless Space Gun".
Sounds like a joke, but it's for real. However, it's no longer in production.
It was probably too expensive and it looked pretty bizzare too, but it predated the Browning design. Google it, you'll see.
BTW, Ljutic is still in business making high end trap guns, the cheapest "off-the-shelf" model going for about $6K. The custom made ones start at about $20K and go up from there. Good stuff if you can afford it. (I can't)