I guess someone has to have a cartridge with sagging shoulders!

Your .25 Roberts is an interesting pick (let’s say “exotic”). Given the length of the case for this cartridge, it suggests a very high initial speed when loaded in “full charge”; which I estimate in the vicinity of 3600 feet per second. But should I be right, then you might expect a fast very high pressure (about 3800 bar just after the chamber) and a fast wear of the barrel that could be noticeable after 200 shots only.
If you reload your cartridges by your own, then I highly recommend you to check carefully the length of the cases before each and every reloading. Better, I would recommend you to case-trim the cases before each and every reloading (beware of a rifle bolt that seems to be a little bit hard to lock!). For the length of cases of small and powerful rifle’s cartridges increase rapidly and, owing to exceptionally high pressures which literally crush and laminate the case’s brass. By the way: better getting rid of the cases for new ones after 10 reloads, so as to avoid broken cases while shooting.

If ever you don’t know what kind of powder and which quantity of it you should use, I recommend to you, as a good and cautious start given this caliber and the length of its case, the reloading tables for a 243 Winchester. Or, perhaps, if you expect more power and a more flat trajectory, then a 10% increase of the powder load. Favor the heaviest bullet for such a small caliber, “boat-tail” shaped for the best accuracy and for keeping the maximum of kinetic energy. I believe that 100 gr.”Sierra” bullets “Match” type (if this brand and this type are still available on the market today, I hope) is the best pick. Favor low-speed powders for better safety… and for saving the life expectancy of your rifle’s barrel!

In my own estimate, the exterior ballistic of your .25 Roberts should guarantee a good accuracy and an exceptionally flat trajectory up to 300 yards and no more (that is to say not a tremendous distance). If your barrel is of exceptional quality, and if you reload carefully your cartridges by your own; then you might get about 100 additional yards of accuracy in the best of cases.

I believe that the aims of the creator of the .25 Roberts was probably a very fast speed and an exceptionally flat trajectory; especially expected for hunting small to middle games in mountains (because games living in mountains can spot or smell hunters from long distances).
Of course and as usual: the greater will be the powder load when reloading, the faster your bullets will travel; but, in revenge, the less accurate your shots will be. Staying under an initial speed of 3300 feet per second would be preferable (but then, yes, of what use would be a so big case for a so small caliber?).

As a conclusion, my personal opinion about this .25 Roberts is about the same as the idea of putting a 500 horses-power engine into a Toyota Camry: a tremendous power in a car that cannot reasonably pass the barrier of 120 miles per hour. Sorry to say it so bluntly. That’s why I put the .25 Roberts in the same exotic category as .220 Swift, 7mm Vom Hofe, .218 Bee, etc., etc.

[…] What your rifle ammo says about you… […]