CH4D is the place to go. I would send then a fired case & Ferrosafe chamber cast. If true custom nothing on the shelf it's not going to be fast. Might think about getting a die blank from CH4D and have your gunsmith run the chamber reamer used for the rifle in for a custom fit. On a single shot not so many rounds loaded over the life of the rifle not necessary to have real hard dies
Something to think about is custom chambers and dies don't always work like factory, which has some tolerance built in to make things work. Case expands on firing then springs back letting it extract. Few thousandths clearance all around. Custom you have two different guys trying to produce something that fits looking for precision rarely comes out working like you expected.
If the case is slightly smaller at the base it may expand to improve fit when fired. When re-loading neck size it only letting the chamber itself set the fit for the rest of the cartridge. Get down near the web were brass is solid very little change in dimensions on firing no spring back. Most single shots have weak extraction, compared to a bolt gun, slightly smaller base is not a bad thing. Sort of like small base Semi Auto Cartridge Dies.
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