Get a machinist to make you an elongated, tapered expander ball with a piece of 7/16" (.4375") drill rod. Taper it from .375" to about .420" and thread it to fit your decapper stem. Use .375 H&H brass and neck it up. Then put your .416 expander ball and decapper rod back in the sizing die and size the cases to create a false shoulder on the neck, just enough so that they will barely enter the chamber. Use a modest amount of medium burning powder and the cheapest bullet you can find and fire form.
I used to use this method when I first bought a .280 Remington about 35 years ago. The brass was hard to find so I used a piece of 5/16" (.3125") drill rod to make a tapered expander ball. I tapered it from .270" to .3125" and necked up .270 Winchester cases. I then put the factory .280 expander rod and decapper pin in the die and sized the cases just enough that they'd just barely chamber and fire formed them. I could have necked down .30/06 cases but didn't want to use them as they have an overall length of 2.494" and .280 cases are 2.540". You can't just neck up .270 cases to .280 as the shoulder on the .280 case is .049" further forward than the .270. A necked-up .270 case would have excessive headspace.
