JD_Zincavage-

Your link to the IAA information supplies most of the necessary background for working with your rifle. I can add that the article by Ned Roberts describing much of the development is found in the January 1925 issue of The American Rifleman. Below I've linked to one of his photos included in that article.

To make ammunition for a rifle chambered for the Niedner .25 Roberts, I would do as Ned Roberts did and start with 7x57mm brass. The 7x57 cases should be sized in your full-length .25 Roberts die, using a really good case lube like Redding's Imperial Case Sizing Wax. Depending on the resistance you encounter, proceed gently. You are going to have to move the case shoulder back, and increase the taper of the whole case body, as well as reduce the diameter of the neck.

The cases that emerge from the FL sizing process should fit the chamber of your rifle. If not, check the cases against your chamber casting.

The problem you may face is determining the overall case length. When you scrunch the 7x57 case to the .25 Roberts form, you will be moving a fair amount of brass, and it is likely to make your cases longer than they should be. In the 1935 article, Roberts wrote that one of the reasons Griffin & Howe proposed their version of the .25 Roberts was that it decreased the amount of neck trimming required on the formed cases. Since you evidently have the shorter-necked Niedner version, you will probably have to trim the formed cases to avoid the problems of a too-long neck. You might contact the maker of your dies to find out what are his recommendations. Hopefully he will have the length dimension of the .25 Roberts Niedner version, and not the .25 Roberts G&H version. (Roberts wrote that the G&H case is about 1/16-inch longer than the Niedner case.) If he does not have a max cartridge length measurement in his files, you might contact one of the knowledgeable IAA persons.

It's possible and even likely that the cases produced by FL sizing of 7x57 cases may not require fire-forming of any kind. After trimming, the cases may be ready to load, just like factory brass. However, until you're sure of this, some initial trials using the cream-of-wheat forming technique may be advisable. If you happen to make and fire a case with a too-long neck, the c-o-w method doesn't produce the possibly dangerous pressures that may occur with a bullet in place.

There's also the question of whether the headspace of cases formed by the FL die is correct. (I recently worked through a problem of a FL die headspace mismatch to the chamber, which produced case head separations.) Some c-o-w firings may reveal any difficulties here.

Good luck.
--Bob
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