I ran a case through the FL die just for you! No form/trim die at all. It’s not as pretty but it would fire form just fine and then look just like any other Mashburn case.
300 Win Mag Winchester brass
FL die, this one is a Whidden
Finished brass
If you were going to go this route I would recommend a die that sized the entire neck, ie a Redding FL or RCBS FL die and not a Redding Type S bushing die.