I came into a few boxes of older 200 grain 35 Whelen ammo recently and experienced 4 out of 20 misfires that required a 2nd strike to fire. Rifle is a custom job with an FN commercial mauser action, Wisner 3 position safety and a Lothar Walther barrel. The fired brass has a nice formed shoulder but the factory ammo has a sloppy looking shoulder. Being a mauser style action it should have a very firm primer strike. I'm guessing the weak shaped shoulder is allowing the case to be driven forward a few thousandths by the firing pin resulting in a light indent. I'll reload some cases being sure not to set the shoulder back and see how it goes as a next step. Check the photo for a fired case should vs factory.