Why shouldn't I use 6.5x55 dies ( which I already have ) to load 6.5x57 ammo? In essence the 6.5x55 sizing die will act as a neck sizing sie on the 6.5x57 case.

It isn't only the body dimension but the shoulder angle that is different.

the 6.5x55 has a 25deg shoulder angle.
the 6.5x57 has an 18deg55min

The 6.5x55 shoulder is also somewhat wider (.435") than the 6.5x57 (.431)

So if even if you try to size the whole
the neck the 6.5x55 die will probably avoid
the doing anything to your 6.5x57's shoulder

proceed with caution, the 6.5x55 die IS shorter
so it can reshape and set back the shoulder if you screw the die down too far, but other than that....

AD

Just be aware that the case body will not be supported (read guided) in the die so you may find runout to be a bit of a problem. To keep runout to a minimum, you can size the neck in two or three stages. Size the neck part way and withdraw, turn the case, then size a bit further. This produces a very straight neck. I do it with my hornet - that case deforms real easy!

Oh, I used to neck size my 303 Brit using a 32 ACP die.