6.5mm/.257 Roberts

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This rifle was rechambered to shoot the 6.5 bullet in the .257 Roberts case. This is a wild cat round and it is easy to reload for.

If you cannot find 6.5X257 Roberts dies you can size the cases with a RCBS .257 Roberts trim die. The trim die has a neck .010 larger than the ,257 Roberts FL die so it works.
You can seat the bullets with a 6.5X55 Swede seating die.

That Mauser made a 6.5X57 Mauser round is a mere coincidence. It is not very close to the 6.5X257 Roberts round. The shoulder has a different location and a different angle.

There was also a 6.5X54 Mauser round (not to be confused with the Mannlicher-Schoenauer round) and there was the 6.5X58 Portuguese round that developed for the 1904 Portuguese Vergueiro.