Please educate me on htis... I want to turn 7x57 into 257 bob.. What do I do?? just run through sizer? trim? what else?? Thanks Marc

With that little bit of change, you probably won't need to thin case neck walls, but I would check the loaded rounds to see what diameter the necks are with bullets in them.

I would at least compare them against fired case neck diameters. The fired case ones will be smaller because they spring back some after firing, but I would check anyway and make an educated guess as to whether they are small enough to easily release the bullet when the round is fired.

Another way to compare would be to measure some standard .257 brass with bullets in it and compare that figure against the brass you form, with the same make of bullets in it. They should be within about .003" of each other, I think, to be be totally safe.

If it was me doing it for my gun, I'd probably go a litle farther and either ream or neck turn the cases so that the ones I formed were the SAME diameter with bullets in them as the factory brass with bullets in it.

As Alberta says, there will probably be no need for neck thinning. It is my experience in necking down most calibers that since factory chambers are usually overly generous, slightly thicker necks actually fit them better.

It is best to trim (and then chamfer) the brass to a uniform length after resizing, even if it is within length limits. Trim to about .01" short of maximum.

One other piece of advice: Depending on how closely your .257 FL die matches your chamber, you may be able to back the die off of the shellholder a bit and still chamber the brass. I advise to screw the die into the press only far enough to allow the resized (reformed, in this case) cases to enter the gun's chamber without risistance. Sizing them further will just result in extra working of the brass and slighly excessive headspace on the first firing. In fact, forming brass from a larger caliber is one way to create a "false shoulder" on which a gun with otherwise excessive headspace can headspace properly.

You don't mention whether you are using new or previously fired 7x57 cases. If they are previously fired, especially more than once, they may need annealing in order to reform properly to a smaller caliber.

Oh yes, just one more thing: Don't use an excessive amount of lube or you'll get hydraulic dents in the shoulders. I like the Redding Sizing Die Wax best, but any properly applied lube will work. If you do mess up and dent some shoulders don't worry too much about it as the dents will shoot out on the first firing.

My experience is the same as Stonecreek's
...factory chambers are usually fairly sloppy as to neck dimensions...which is why I said I felt pretty safe when being within .003" of your factory diameter brass on necks...

But, that's factory guns. I notice you have posted here at AR fairly often, so I wondered if maybe you might be at the point where you have a custom-barreled rifle. Some of the better ones of those DO NOT have sloppy necks at all, and a few even approach being what target shooters call "tight-necked" chambers.

With those it is very important to not have necks that are too thick as a too tight neck which doesn't release the bullets easily can drive otherwise okay loads and their pressures way above where you want them.

Thanks for all the great tips guys. It is a factory Ruger -77 I've set up for my 15 YO grandaughter. I have a bunch of 7X57 brass and was in hopes of using it.. Again Thank you.