Does anyone offer such a tool which would allow for decreasing headspace? 

If not, I have an ideas how one could be designed.

In my process for making .25-20 SS from .223, I use a slightly concave punch during the major swaging operation to curl the rim forward a little, making it functionally thicker.  Not a new concept, I learned it from an old gunsmithing article.

Case rim thickness varies by brand. Sometimes as much as .010

Gentle peening the rim forward and then cleaning up the rim in the lathe is a quick way.
A die, a rod inside the case and a hydraulic press is another way and gives a nicer looking rim but a lot more work.
Otto

art_ruggiero wrote on Apr 7^th, 2023 at 2:02pm:

I like the idea of a tool to modify each case that way, if the rim thickness varies, you can dial in each case.
My head space is about 0.005". Not sure this is excessive for a 40-65 and blackpowder use. I'd like to experiment though and see if I see any improvement.

This is a great idea. The die and punch would be a simple 30 minute or less lathe job. I don't have a need for it at present, but will file the idea away for future use 

Here are the tools I made to measure case rim thickness and also to increase the rim thickness. I copied the rim thickness increaser tool from Texasmac. Thank you for the idea!

Texasmac, I have a question, did you use a press to increase your rim thickness? Using  a punch doesn't work so well.

Hepburnman
You may be able to move thin rims forward with an Ann Arbor press but to form cartridges like the 43 Mauser, you will need something stronger, like a Harbor Freight 40 ton hydraulic press.
You may have to modify your die to prevent locking the rims into the die and you will probably want some indication of ram movement, to gauge brass displacement.
If you don't have access to a hydraulic press, you can get useable cartridges, by careful hammer peening of the rim. A jig to hold and support the rim, with clearance to allow the rim to move forward, worked surprisingly well to increase rim thickness .010" on a 8.15X46R cartridge.
Otto