257 to 7mm


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tahoe2
March 2, 2013, 11:06 PM
I heard it over and over; the 257 Bob is a necked down 7x57 mauser,
so I have a box of brass(257) and I was going to neck them up to 7mm.
Has anybody done this successfully ??

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rcmodel
March 3, 2013, 12:08 AM
The .257 Roberts is based on the 7mm Mauser case.

But in addition to necking it down, the shoulder is moved foreword.

If you neck down 7mm Mauser to make Mr. Bob?

I believe you will have some excess headspace to deal with somehow while fire-forming to blow the shoulders out.

Course, I could be wrong?

But you can do the Chinese Algebra by comparing the two cartridge drawings & doing the math in your reloading manual.

I'm to tired to do it tonight.

rc

NCsmitty
March 3, 2013, 12:22 PM
Here are the drawings from stevespages.com, and as rcmodel mentioned, the shoulder may need to be pushed back a bit to fit your 7mm chamber, depending on it's condition and dimensions.


NCsmitty

beatledog7
March 3, 2013, 08:14 PM
Have you considered letting the Bob brass go to a new home instead?

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