I posted this info on another site, and thought I'd mention it here.  You CAN make rimmed brass from rimless.  I didn't believe it at first, but now I know it can be done.  It requires at least a 5 ton press and a 7mm FL die.   The 7mm neck provides the sizing for the new case.  You also need a lathe.   You push the brass back almost to the rim, then you trim off the excess with the lathe.  Then trim the case to length.   It's important to push the donut all the way back so it has a sharp ridge, not a flat ridge.

I made a video of the process of how to do this. I am no video guy, and my knowledge of how to use my video editing software does not extend beyond the absolute basics, but hopefully this rather hokey video I put together will give you a picture of the process.
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This will make brass that is very strong and should last a long time.
Any questions or more details needed, let me know.

Unlike some brass, 223 is readily available and inexpensive if you want to experiment.

Well done!

Brian added one little fillip that I never thought of, which is using rosin to help keep the case from slipping on the driver spud while turning down the "doughnut".  Saving that for future reference.

A reminder: This brass is much thicker than OEM, so published loads have to be backed off a full grain more than usual to start.

The 7x57 Mauser is much to be preferred, because it has a relatively shallow shoulder angle.  It's that part of the die that does all the forming.  A steeper angle would require more pressure at the end, and the face of the case takes quite a beating as it is.

Oh, and since I see uscra112 is on here, I gotta give him big props, as he's the man who taught me how to do this.  He also suggested that a more effective driver for the case on the lathe would be, instead of trying to drive the case using the live center at the mouth and some rosin at the head, to make one that is held in place at the tailstock and goes into the case and jams up against the inside of the case head, pressing it hard into the little aluminum driver held in the chuck jaws.   That should let you take off more brass with each pass, and speed up the lathe work.   I plan to try that next batch.

Two things.  First, I cut a 'driver' in the collet that slightly wedges into the primer pocket. Therefore not needing rosin.  Could be cut to ensure that hte primer pocket is not undersized (like a primer pocket swaging tool).  One pass with the cutter does the job on bringing the brass to size.

Secondly, these are sections from early on, to see how far the forming must go down the case.  Note that if it is not far enough as shown - the brass is far too thin just above the base.  These were done on a Rock Chucker.  Next attempt will be to try to do it in two passes taking it down a little at a time so it can be done on the Rock Chucker.

A couple of years ago Phil sent me down this rabbit hole and I discovered it's a very deep abyss.  It's not that anyone will take it this far but I had to.  Here's a photo I just shot with some descriptions of what I made experimenting.  There are a considerable number of dies to make to get to the end results here and the frustration level goes up considerably. 
 
I have used the swaged 25-21 brass I made with probably 10 reloads on it - in itself it's a fragile case to reload and I've found the 25-25 even worse.  Anneal too much and they crumple, don't anneal and they split.  The Vickerman seating die (previous post question) works well for these... After dealing with these longer cases, the 25-20 are a breeze.  I've made nearly 100 of these and they function and shoot quite well.
 
I really appreciate the video Brian, the press you are using is a necessary item and if I do any more I'll go that route.  I currently use 5 ton arbor press - it works but it's a workout.

* I labeled the internal nibbing tool a swage in the photo, struggling with the pencil this morning... There's a die that went with this that gets chucked in the lathe - it worked but it was slow and tough to repeat dimensions.  The arbor at the bottom of the page has marks for lengthening for 25-21 and for 25-25.  At a glance you can see where you're at.  The roll, replaces a knurl in a knurling tool, a little lube and the brass grows quickly.

Phil,
Here is a couple pictures, the one in motion was dated March of '20 so it's been a while since I've touched this.  The second photo is of the actual arbor, I smoked it and filed the profile to match the inside of the case as close as I could get it.  It has a line that I take the brass to for 25-21 then up to the ridge if I'm doing 25-25.  I did this in a 3 jaw chuck and looking at the shape of the portion that was chucked on, I had an issue at some point, appears to have spun in the chuck - I can't remember that happening and can't believe that the brass would have stuck to the mandrel either...
I have a LW that I re-barreled in 25-21 but never finished it... wood I think.  Now that this has all resurfaced I may have to change direction a little...
thanks for sharing this process with me back then.
Greg