Never "pinch" a cap because it doesn't fit properly. Throw the tin away, (or lose your hand or trash your gun)?
How pray tell is either one of those going to happen from a pinched cap? And don't you dare say chain fire. In a chain fire the only chamber that has any force is the one in battery. The rest are very weak, Without bore pressure the balls only go a few yards and drop. Your arm might get peppered with cap fragments but they don't normally break the skin.