The .25-25 was not a target cartridge, according to Ned Roberts.  The .25-25 I put together on a Low Wall action has not shown stellar accuracy, either.  The loads I use are pretty stiff and use the Remington jacketed flat point; definitely not for a 44.

You might try black powder, or, alternately, try starting loads for the .25-20 Repeater.  My lead bullet loads in the .25-25 never rose above the ballistics of the .25-20 standard loadings anyway.

About 6.5 grains of #9 with an 86 grain bullet (as fixed ammunition) works pretty well in my .25-21.  No reason to suppose that it wouldn't work in the long version.

Ditto 3.0 grains Red Dot, for a 1200 fps load.   

The Red Dot did not seem to be position sensitive, but the #9 load may have been.  Neither load has been really wrung out; my .25-21 is a 44 1/2 which has gravel in the trigger pull, and so far I can't seem to find the correct round tuit that will allow me to fix it.   Model 44s that need my attention keep popping up. 

The Red Dot load is close to 15,000 psi fps,  according to Quickload.  Maybe back it down a bit for the 44, unless it's a nice tight one?   2.5 grains would be just about subsonic.  

7.0 of 2400 would be safe.   I tested 2400 in the .25-21 and wasn't happy with the SDs, though.    Ditto 8.4 of 4227.