Same thing happens with mine. It's done it with handloads, too. Wanna hear my theory?
When the primer lights off, it backs out of the case a little because of the pressure it produces. As the case fireforms to the chamber, it slams itself backward into the breech face, leveling itself off with the primer again. 7-30 (and maybe .30-30, too) brass may not be allowing itself to move back to the breech face to level off with the primer again.
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)
Joe Kool is correct, at least from my experience also.
I fireform all of my Waters cases from 30-30 brass. I size the 30-30 case in a 7-30 Waters die JUST ENOUGH to allow it to chamber. The secondary shoulder holds the case in the chamber and against the bolt/breech face so it doesn't move upon impact of the firing pin. This case is then a chamber match--almost. I then just neck-size this case for the next reloadings and there is no head space problem that I can detect. Good-luck...BCB
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