I have a 12" with a .308 bore that will drive tacks (Bullberry)! However, it's a handloader only since it has the .308 bore, no factory ammo allowed. If you have one built, I would recommend the .308 bore since there are so many different 30 caliber bullets available to the handloader. As a "wildcat" with a .308 bore, it's a fantastic little cartridge. Most reloading die sets come with both a .308 and .310 expander button, so you can load for either bore.
If you're not a handloader, I wouldn't recommend this chambering at all, you'll probably be dissapointed if you want it to shoot cheap ammo. Also, the factory ammo that is available is on the borderline of being too much for the Contender frame. The pressures of the little 7.62x39 cartridge can get on up there on the C.U.P. scale.