In his book "Handbook for Shooters and Reloaders Vol 1" PO Ackley refers to a 25-20 wildcat that wrings another 15% velocity from the case. I believe the bullet is the same. Anyone know anything about such a critter?

Be easy enough to do, but why would you want to?

.25-20 WCF has more neck than it needs and is a rimmed case. Be easy to move the shoulder forward, sharpen the shoulder angle, and reduce the body taper, but, at most, you would make it a 150 yard varminter rather than a 125 yard varminter. Less than that if you are stuck with RN bullets in a lever gun.

There are lots of nice .25 wildcats that don't take messing up a scarce gun in .25-20 WCF. Were I you, I'd look at .25 Copperhead, .256 Mag, or .256 Max.

.25-20 brass is thin, and, therefore tends not to last long. Fireforming it wouldn't help. How about opening/fireforming .218 Bee brass ?
Another idea would be Wooter's Tooter, named by John Wooter's. That one is the .222 opened up to .257. E

The easiest way to improve the .25-20 in a bolt gun or single shot is to feed it the long discontinued Speer 60 grain cone points. Next best is whatever 75 grain spitzer that comes to hand. The 75 grainers (and I think I used Sierra) give a little expansion on chucks at 75-90 yards. I haven't tried them over that, but I have serious doubts that you would get any expansion at all at 125 or more.

Thanks for the info, folks! This post came about as a result of a 25-20 shooter telling how he loved the cartridge and such. I mentioned the PO Ackley version to him but could't look it up in the book since the computer is home and the book is in my workshop a couple miles down the road

You might be thinking of the .255 Dean Wildcat from the early 1940's. It was based on the 25-20 case but has straight walls and an abrupt but rounded shoulder. Holds 17 grns of 4227 powder , with bullet weights up to 70 grain listed. Factory 25-20 cartridges could be fire formed in the rechambered rifle.
Hope thats of some use ...