Oh, you're only going to shoot mid range.

Even more options then, especially since you do reload and don't mind the easier wildcats. I like many others too, but...
My personal first/second choices by caliber would be...
17DT/17MachIV
20 Practical/20 VarTarg
22SB/226JDJ
6X47/6SB
257JDJ/257JDJ
6.5SB/6.5JDJ
Can you tell I too really like wildcats. The 17DT, Super Bower's and JDJ's are rimmed cases naturally.
As you'll notice, the hands down quarter bore IMO is the 257JDJ. Very fun barrels, I've had two SSK's and they'll shine big time at 300 yards.
Picking only one would be hard if I was only going to shoot to 300 yards. But if the shots could/would get much longer the 6.5 SB is my obviouse choice (it's good to 1K).
Also agree with Ken on the barrel length. You can always shorten a barrel if you find you don't need it, but the extra velocity does help make the Contender pistol a short rifle ballistic wise, which will be important at 300 yards and beyond if the targets are small.
YMMV