You tube :
Fire forming 375 win from 30/30 brass ....
This guy Glen Larsen shows how in just a few easy steps ..
He uses 7 gr of unique powder in a primed 30/30 case , packs it well with tissue paper / grits .
Fires it his 375 Winchester contender ..
It conforms the case to .375 dimensions , all you gotta do is trim to length..
I've not tried this , but looks simple enough ..
Bob