Thinking of rechambering my 1909 argentine mauser in .257 Roberts to 25-06. Is there any reason this won't work?

A 25-06 reamer will clean up a Roberts chamber. If you have a shorter barrel and already load the Roberts to modern pressure not sure how much you will actually gain.

It is a significant change if you want to use bullets heavier than 100 grains. Up to 100 grains there is no advantage. There could potentially be magazine length issues with long heavy bullets. Ditto previous post.

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It is a significant change if you want to use bullets heavier than 100 grains.

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It is a significant change if you want to use bullets heavier than 100 grains

YEARS ago I did exactly what he is asking about. I had a 22" Roberts throated so I could seat the bullet long. Going on memory I think is was the 120 Speers (GSlam?) I remember the gain as being in the fps range.

Of course longer barrel would have helped. Also who is to say I was loading to equal pressure.

Take a Roberts in a LA throated to allow you to seat it longer than std. Load to 6mm or 25-06 pressure and it gives up VERY LITTLE to the 25-06.

As you stated I love the 1909 mag. But have had issues with the box length. A file helps