Expanding 57mm brass out straight (or .30/06 brass for that matter) is one of the simplest of case forming jobs. One only needs a .44 magnum expander die, then grind a straight taper on the end of the expander button. This will then enter the .7mm or 8mm neck, and with a bit of Imperial die wax in the inside, expand them out to straight in one pass. I do this for my 9.3x57 and with .30/06 and .35 Whelen brass for my .375/06IMP. So simple it is absolutely ridiculous & fool hardy (a waste of money) to buy someone else's cases.

The cases are then simply necked to create a crush fit in a FL or neck sizing die to fit the chamber, whether it is a 9.3x57, 9.5x57, .35 Whelen or .375/06IMP.

The reason for necking straight is to adjust the headspace of the 'new' brass to perfectly fit the chamber of your rifle, so there is NEVER any stretching at the web. If not formed, there will be stretching, which simply continues until eventually separating. Simply necking up will create excessive headspace in many chambers, and up to .007" slop in 'factory" spec. chambers.

I also used this method for forming my 9.3x62 out of .30/06 brass way back in 1982 - still have that brass and it is still being used, after well over 12 rounds per case.

As to this statement "Others say this is a very bad idea, as the brass is likely weakened at critical points during the stretch." - that is simply not true, ie: bullshit."