Primers
I think what is happening is that factory rounds are sized to minimum headspace to function dependably through lever actions. The chamber in your contender may be maximum. If so, the case will have a few thousands gap between the face and the breech block. The firing pin will drive the cartridge foward into the chamber. Upon ignition, the pressure will cause the walls of the case to grip the chamber walls, but the gap between the case face and the breech block still exists. Then one of two things will happen. If the pressure is high enough the case will stretch just in front of the web, or if the pressure is not high enough, the primer will back out. Nothing can be done about it with factory ammo, but if you handload, the cases can be sized to give minimum headspace . For the 7-30, use a 30-30 case and size it to 7-30, but size it long, so that the action will not close. Then slowly turn the sizing die down and keep tring the case in the action, just untill the action closes easily. This will give you a zero, or near zero headspace. It may take a few firings for the factory cases to lengthen for the sizing die to make contact with the case shoulder. At this point if the action does not still close easily, turn the sizing die down a little, untill the action will close easily. 8)