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
[Linked Image from i.imgur.com]

[Linked Image from i.imgur.com]

FL die, this one is a Whidden
[Linked Image from i.imgur.com]

Finished brass
[Linked Image from i.imgur.com]

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.