I picked up a bag of 284 win brass today and cut a few off at the sholder. I then exanded with a 45-70 die and seated the variety of Lee 457 boolits.

left to right
45-70, 500-2.515", 450-2.515", 405-2.255", 340-2.247" and 284 win case.
The cases are cut to 1.8". I would imagine that we might be able to expand the neck with anealing and several steps.
Do you think that they would reliably headspace off of the case mouth?
I'll go and feed some through a Mauser and see what they do.

Okay, I just got back from the toy room. I tried them in an 06 and the long ones fed fine but the short ones wanted to nosedive in the magazine. In a 35 Whelen and they all fed fine. In a 308 and only the longs ones fed. In a 338 win the all fed great with the rounds just jumping into the chamber they extracted fine even with the boltface and extractor opened up. The 7x57, 8x57 and 6.5x55 were kind of burried, so I didn't try them.