hop,
As I mentioned in a PM, you should size the 30-30 case down the neck a bit at a time until it will chamber. I don't think sizing it all the way down is the correct way to go. You will most likely end up with a headspace situation and the case will not formfire as well as can be expected. The Contender should close with a bit of resistance yet still allow the trigger to be cocked and able to fire the case...
Use the secondary shoulder to headspace the case in the chamber. Yea, I know that the rim is the true headspace on a rimmed case, but the shoulder will insure a good tight fit in the chamber when it is fired. Thusly, the case will fireform better to the chamber...
Good-luck...BCB