Look for posts by user Fox. He recently had a series of threads about using 17hmr cases to make ammo for a 25rimfire.
One of the biggest issues was trying to find a .251 bullet if I remember correctly.
Hey, thanks for pointing to my post, I have to do a little more work on this.
It is not 17 HMR, it is 17 WSM, which is based on the 27 cal nail gun blank. Remember that nail gun blanks do not fit rifle chambers for a reason, so you have to look at the specs.
The 17 WSM is about 7 thou under sized to the 25 Stevens but the rim is the correct size. The rim is thicker though on the 17 WSM and the primer is harder, I have a Cooey Canuck and was able to get a bolt with a custom firing pin installed, I still need to fit it properly for the gun for headspace but this allowed me to go from 4-7 strikes on the primer for a bang to 1 strike and bang.
The key here though is that you CANNOT USE THE EXISTING POWDER, you do not know what powder it is and what sort of pressure curve you have, the 17 WSM is a modern gun running 33,000 PSI, the 25 Stevens is originally a black powder round.
That being known though I take a 17 WSM, use a small pipe cutter and cut the case just below the shoulder, I can then knock out and sell the .172 bullets. I then dump out the powder into a collection pan for use in the garden as fertilizer.
The case will now need to be straightened and flared for a bullet, essentially sized, I have 25 ACP dies for this, the powder through die has an expander on it, this works well. The shell holder that fits best is for a 30 Carbine. I ran a test load with 9gr of FFFg, I wanted to start low and compare to existing original short and long rounds.
The bullet is a .257 cast 72gr pure lead bullet sized down in a custom sizing die to .251. The die knocks off all the lube grooves, so I take the bullet and roll it through 2 coarse files to knurl it, then roll it in 50/50 beeswax and Crisco, this seems to work fine for me.
The only batch I have made so far involved seating the bullet directly on the powder but I now have a compression die made up for the powder, seating directly on to the powder causes a lot of deformation of the bullet nose and the weird velocities over the Chronograph proved I needed to bulk up the consistency.
I shot 5rds over the Chronograph, ended up with about 800fps with a 72gr bullet, this is with a low starting powder weight and coarser powder than what was originally used, they used to call rimfire black powder "dust", so going to get a can of FFFFg to try.
The gun cleans up like any other BP gun, take the barrelled action off the stock, hold it with a Silicone oven mit and pour a kettle of boiling water through it, no corrosion at all and not a lot of fouling when shooting a few rounds.
I have a spacer now for more consistent seating depth and the powder compression die as well as that new bolt and firing pin, looking forward to making a box of 50 for about $20

, that is shootable, just don't ask me about how much investment in the tooling, ha ha ha.