NitroX:

Seeing's you are Down Under and all, why not a wildcat based on the .303 British case? The Germans have what is similar, the 5.6x57 {8x57 Mauser necked down for shooting heavy weight bullets for roedeer}. A .303/.22 would address "CCCC" {Classic Commonwealth Case Criteria

}, would provide a case big enough to provide plenty of pressure-reducing volume and the barrel could be rifled with a quick twist for the heaviest .22 bullets.

In addition, the .303 is of course rimmed, meeting another double criteria and a light 20 gauge shotgun would I think be a suitable start for such a gun.

You might not reach the same velocities attainable with the German 5.6x57 due to the use of that cartridge in strong bolt rifles and your desired use of your .303/.22 in a double, but the velocities you could attain should exceed those produced by the .219 Zipper at Zipper pressure or equal them with less pressure.

I change my vote.

First place; .303/.22!

Second place; .219 Zipper!