As a reloading proposition, making 25-20 from 32-20 is no big trick. Starline has .32-20 brass on a yearly basis and .25-20 dies will resize it with few losses. After reforming, I trim mine to square up the case mouths and reload normally. Here's how I do it:
Forming 25-20 from 32-20 brass
1. Round case mouth, chamfer & then anneal.
2. Lightly lube with Imperial Sizing Wax.
4. Use 25-20 Seating Die to set shoulder back.
5. Degrease then use graphite inside neck: 25-20 Sizing die w/o decapper to final form case.
6. Re-anneal, square mouth & chamfer etc.
7. Re-anneal after 5 reloads.
Bullets: Lyman's 257420 GC sized 0.258" cast from wheel weights (about 75 gr.) will withstand muzzle velocities up to 1600 fps in my gun.. RCBS' 25 cal. plain base cast bullet at ~95 gr's is good for lower velocity loads. In jacketed bullets, the old Remington 86 gr FN soft point is good for ~ 2" at 50 yds in my gun. Speer's 75 gr FP and Hornady's 60 gr FP at so-so in my gun, but Marlin users report good luck with them...difficult to find though.
Powder: For ~1000 fps, Win 231 does well. At higher velocities, 4227, SR4759 and 2400 do well especially with cast bullets. I've not used Herco or Unique with my gun, having found good accuracy with the preceding.
The .25-20 is a good cartridge for woodchuck sized critters and can be loaded down with cast bullets to mimic .22 LR or Magnum. And at velocities from 1100 to 1600 fps, it's a great walking about, wood's loafer's gun. My M-65 Winchester will keep most decent loads in ~1" at 50 yds with a Marbles Tang Peep mounted.
It's a round that's a throw back to earlier times, before the bottle neck, high velocity varmint cartridges began to dominate. In a lever gun with a good barrel, the .22 Hornet, .25-20 & .32-20 guns can be very useful and with a much quieter report. Scoped in the case of the Marlins, or peep sighted on Winchester & early Marlin lever guns or one of the pumps, the .25-20 is a lot of fun.
Best regards, Rod