I've machined brass bar stock for these in the past and they didn't work out so well.  A shooting acquaintance gifted me with some from a company that turns them and they're holding up better that the batch I made but I had an idea... 
A member here shared a process he used to make 25-20SS and 25 Lovell.  I came up with the brilliant plan to try and make 25-21SS the same way.  I started with 223 brass the same way and as they are - without nibbing - they turn out about an eighth inch short of the desired dimension.  I did try making some using a 223 basic brass that Starline produces, the length comes out about .050" short of size but there's a couple extra steps and another die (or two).
Here's a picture or two of the carnage I made today getting to the end results.  I started with about 25 pieces of brass to get eight operational.  I'm sure the percentages will get better as I refine the process.  Hopefully more to post in a while. 
Greg

The turned cases were from RMC and after doing a recent search at Sir Google I did find a supplier that is supplying round brass now specifically for cartridges - probably the same stuff as before but a different spin on advertising?.  The last time I looked all I could find was bronze sheets for drawing or forming and they touted it specifically for cartridges.  The 260 grade is what I used before...
Following a deep dive research a while back and some lengthy discussions with a metallurgist a time or two - turning is not the best option.   
As one may be able to see from my pictures, the case wall thickness from the forming process is extremely thick, probably .06" in front of the head extending for better than .300" towards the mouth.  The nibbing process would thin this a minute amount, .005" on the thickness and give me .200" in length?  I'm making some arbors for this and hope to have more details to share soon.
One more thing, the member that was making the Lovell brass mentioned that his case life was very good and he was shooting these in a higher pressure application.  I'm shooting these in a Ballard and a Stevens 44 where pressure needs to be low and velocity isn't even an interest - maybe approaching 1200fps.  The case capacity is considerably reduced right now but I can work with that real easy.  When our weather improves and time permits I'll get out and gather some more range data.
Thanks for the interest.
Greg

I have a bunch of brass from long ago that was made from 30-06.  Seems like a lot of work, but they hold up very well.   
Bruce

rmc wrote on Mar 9^th, 2020 at 11:57am:

Thanks for the insight, I made a die this am before the chaos began, and a tool for the inside is almost done, maybe tonight... I started making a mandrel for the basic brass and then I was going to make a smooth roller for a knurling tool so that's similar.  I like the idea with the tubing cutter.
Greg

Greg,

Did you try annealing the RMC cases before shooting?

Dick

Dick,
I did anneal the cases from RMC when I got them, the ones I'd made I'd annealed once but I don't recall all the specifics but mine did fail within a couple loadings.
I spent some time tonight making a few more cases.  I tried the internal nibbing process- that didn't go too well.  I then made an arbor like rmc suggested and a roller for a knurling tool - still had a pile of scrap but the technique of annealing when and how mattered some... not too much before pushing the case through the 7mm die and then a small amount of annealing before starting the staging down to finish.  I have five finished to the length my rifle chamber needs (2.065") so if I can remember all the sequences I'll make some more next opportunity.
Jim, I tried making a couple pieces that would be the 25-25 length, 2.37" - not much luck there but with more experimenting you may be able to come up with a few...
Bruce, I don't have a 28-30 - yet... but I'm sure there will be one in the future.   I'll have to research this some.
Greg

Wow! That's fantastic!!
I'm guessing you're tool in the tail stock fits loosely in the primer pocket and has a hole in it to accept a decapping pin in the body support?
Keep up the great work and thank you rmc for bringing to our attention this method of reforming brass.  Brilliant.

I remember seeing an ad for modern caliber cases with a steel head and brass body . Maybe brass 223 case threaded internal and a steel head with a male thread would work .
Dales