416 Strausse, 416-03, or 416-303BR

[BlueFlyer]

One pattern was a higher fail rate in brass that was not initially annealed.

Annealing is key to forming brass and minimising splits. I made up an induction annealing machine out of bits bought of Aliexpress for less than $150. It is worth its weight in gold.
Fully understand why you have based your cartridge on the 303 case given yoyu are using a P14 action, althought you may need to do something to accomodate the larger diameter projectile at the front of the magazine. You have your reamer now, but it would have been interesting to go down the Epps path with an improved profile to the chamber.
I have gone down a similar slippery slope but based on the 348W case. 416/348 AI in a Matrini Enfield. After I chambered a 416WSM and the 416/348AI using the same reamer. I have had the reamer reground by PTG to 338/348AI for another project on a Siamese Mauser action.
Very happy to see other prepared to experiment.

 

[odonata]

What I am most interested in is reloading data. A starting point!

Since there is so little information available on this particular wildcat, I think you may be on your own & starting from scratch. If anyone developed anything, as far as I can tell, they weren't big on sharing the knowledge.

As an FYI, I would like to add the disclaimer that I have never developed a wildcat. It's something I've been curious about simply because I enjoy shooting but I want to be clear that any "suggestions" I might offer are based on possibly faulty intuition & not years of actual experience.

The original drawing for the 416 Strauss that I had seen had a case that was 2.215" / 56.2mm. Earlier in this post, I think you said your cases were 2.165" / 55mm. The single article about this wildcat said that the developers were getting results of 300gr @ 2,500 fps & 350gr @ 2,300 fps using a South African magnum pistol powder, Somchem S265. Why this powder was chosen is not explained so it's unknown if this was an informed choice or based on some kind of personal preference. But the results are promising if they're accurate.

Looking around for similar powders over here, a quick AI-based query (which might be as unreliable as me) gives a few suggestions:

Looking at a burn rate chart seems to verify that these powders are close to S265 when sorted by that characteristic:

So the next step I took was trying to identify a similar sized case, that shoots similar weight bullets that has posted loads for the above powders. One that looked promising is the straight-wall 444 Marlin with a 2.225" / 56.5mm case shooting a .429 bullet that weighs 300gr. The speed is lower by several hundred fps (possibly because it was intended to be shot out of a lever action) but it might be a good load to work up from if you can find one that doesn't already have a really high fill volume. I will let more knowledgeable posters weigh in on whether this is a good idea or not.