Fireforming question!

[Tikkamike]

I have some brand new Nosler 300 ultra brass so my question is can I fire form it in my 375 rum? or will it become too short? and will it make the necks too thin going that much bigger? and when using brass with extremely uniform necks like nosler for necking up does the neck stay uniform?

 

[Mikecr]

I don't have either. But I can see the neck of the 375RUM is way longer than the 300RUM, so I assume you're talking COW fireforming.
The shoulders are the same angle & both cases the same length, there is no reason for the brass to shorten more than usual with fireforming.
There is also no reason for uniformity of neck thickness to change with expansion.