I have about 90 cases each of 6MM Remington brass and7x57 brass once fired. Which case is the easiest to size to 257 Roberts?

I have necked up 6mm brass to .257 with no problems. If you neck down 7mm brass you MAY have to neck turn the brass necks if you have a tight chamber.

Hip

The rifle is a Ruger UL and though it is a fine shooter I don't think it has a tight neck. I am going to use the 6mm brass as I still have a 7x57 and you never know when it will come out of retirement.

I have made them from 25-06 and 7mm without turning the necks; for a Ruger 77 that must have a large neck; it has a very long throat. I can seat bullets so they barely are held in the case and use 25-06 load data.

I've used both and either works fine. The .244/6mm shoulder has a slightly sharper angle than the Roberts (and 7x57), so it theoretically won't fit the chamber as well. However, I've found that difference to be de minimus enough that it can typically be disregarded.

As between the two, I'd rather neck down than up, but considering the desire to save the 7x57 for an existing rifle I would probably also opt for the minor opening up of the .243" neck to .257".

How about you send me the 6mm and I'll send you some 257.......

I've always had better luck necking up than necking down.

For example, necking .375 H&H down to .300 Wby, results in "folds" in the shoulders.

Powder Valley, Inc shows to have Nosler .257 Roberts in stock

quote:

Originally posted by buffybr:
I've always had better luck necking up than necking down.

For example, necking .375 H&H down to .300 Wby, results in "folds" in the shoulders.

I've found that changes in either direction are best done in small increments and in gradual slopes. If I was necking a .375 down to .30 I'd use a die to reduce it first to .33 then down to .30 but both would be with an interior angle of about 20 degrees or less. Considering the expense of forming dies, probably financially ahead just to purchase the correctly sized brass.

One of my suppliers is showing a large quantity of Remington .257 brass, so I would expect it will start showing up on the market.

Nosler, Remington, and Winchester all make the correct brass, and all is designed for +P loads.
Keep looking around at the usual suspects: Grafs, Powder Valley, Mid South, Gunstop, etc. Your ship will come in. I had a Ruger 77 tang safety in .257 Roberts AI. Great dear, antelope, and caribou rifle.

Either will work, I think you answered your own question in that you have a 7x57...

Necking up or down works fine, just another step with the 7x57 meaning outside neck reaming may or may not be necessary. I don't believe in inside neck reaming under most circumstances.