Sorry, no direct experience, but I have some entries in my notebook that may be of use. The cartridge is of interest to me for another project I keep in the back of my mind.

Handloader No. 188 (sorry, I didn't record the date) had smokeless loads for both the .25-25 and the .28-30. Sorry, I didn't record the range of bullet weights, only that the author, Greg Matthews, had no trouble with smokeless loads in the .25-25. (He did with the .28-30, which he attributed to the long case fired with a small rifle primer).

The Single Shot Exchange has had some articles over the years. Some have reported trouble with fps swings in these slender cases. A reprint in SSE from a 1905 magazine reported glowing success in switching from BP to smokeless in the .25-25.

From the case description -- a cylindrical interior -- I estimate a capacity of 28 gn. Weighing a sample I have of Bertram brass, I estimate a bit over 31 gn; I don't have a resizing die, so I can't make an actual water estimate.

The Bertram brass is basic and must be resized. I recall (but did not record) one shooter in SSE complaining the Bertram stuff was wide at the base and extremely difficult to full length size. My old UMC sample is .301 at the base, and my Bertram sample is, sadly, stored back in the US. Rocky Mountain offers lathe turned .25-25 brass as well.

About 26 ksi would scale breech thurst nicely to the .410, which my notebook says runs at 14 ksi. Both the Powley Computer and QuickLOAD predict 1700 fps is possible at that pressure, and both hint that a reduced charge of 3031 or 4895 would do the trick.

If you can go to .22 Hornet pressures (low 40's), you should get close to factory .25-35 speeds with the 117 RN throated long, ie. .3 inches deep.

Hope there was something of use in this ramble.

Karl