Just use imperial, and run them through a f/l die, and out pops use able 257 Weatherby brass?

Yes

270wby happens the same way at my house btw

Cool.
Thanks guys.

Visual:



Not my picture, but you have 7mm RM on left, 7mm RM run thru 257 Bee FL die (obviously not FF'ed yet) and factory 257 Bee on right. Necks will be shorter with the 7mm RM cases.

Yeah.... I see that.
Any adverse effects ?

Nope, done hundreds of them and they have worked perfect.

Great !
Thanks.
Do they fireform into the rounded shoulder ?
I've made 280ai out of 280, but it's kinda the same as this, but not really.

None made hundreds of them

Used one shot spray lube, and they chamber just fine.
Now to load a few, and see what happens.

I got 257 wby brass if you're looking.