Well, as .303 is not on the list

I have to agree with Yves but I would choose the 7x57R because there is a greater availability of 7mm bullets.

For a first double, you want something that you can shoot ALOT whether at the range or hunting and for me at least that rules out anything 9.3x74 and up. A 9.3x74 would be my choice for general hunting, but not for shooting ALOT and really getting to know the rifle and handling a double.

Another important factor in owning a double is relaoading and working up loads that shoot to the regulation. Therefore having a wider selection of 7mm bullets adds to the choice, variables and fun! Plus its cheaper on both bullets and powder than a .450/.400, which would clearly be the best choice for a first DG double.

Just a few things to consider. Its not an exact science and its the variables that make up part of the fun!