Just a question....how did they make cases for this chambering 'back in the day' ?
Short Answer:
Good Question! I was hoping so and looking to find someone with a few lying around or a sizing die I could buy, rent or borrow. Nothing - Nada - Zero - Zilch!
Part of the problem is that as near as I can tell - the cartridge was never "standardized" and this caliber seems the least popular of his offerings so few/no "box of old dies on Gramps shelf in the garage"! My suspicions are It is likely that Mashburn sold dies and cartridges with his rifle sales. Dies probably could have come from him, or perhaps from any of a number of die makers back in the day.
Much Longer Answer Below::
Making cartridges for this caliber "350 Mashburn Short magnum" is addressed in two books I have,
Parker O. Ackley "Handbook" (1962) and "Handloaders Manual" by Donnelly (2011). Seems straightforward.
"Seems" is a word with many variations it seems!
Ackley
States on page 478 to use 300/375 H&H Cartridges necked down and shortened. Then goes on to say this is essentially the same as the "35 Ackley Magnum" and use that info for the Mashburn. Further info on page 475 for the 35 Ackley Magnum mentions forming, shortening and fire forming and lists several loads. Interesting but not terribly useful (except the loads!)
Donnelly
On page 263 there is dimensional drawing, load data (1) and case making info. Case making consists of using 300 H&H, expand the neck, trim the case, anneal the neck, and then the kicker -
Full length Size in Mashburn Die! Check trim and chamfer the edges.
Me
Surely there is an easier way! I have borrowed "on approval" as set of 35 Ackley Magnum dies. The start of the shoulder is the same dimension for both Ackley and Mashburn - 2.05". I started with a nearly identical cartridge - the "358 Norma Magnum" which was not available "back in the day". It is also based on the 300/375 H&H cartridge and already has an upsized the neck! I bought 20 cases.
The Norma brass runs through the Ackley Die easily - but seems to be expanding the case diameter just below the shoulder/taper. Another issue is the length of the taper - Mashburn uses 0.06 and Ackley 0.11. I cannot tell for certain where things are getting hung up in the chamber - need to get my candle lit and do some soot testing!
My next plan (besides soot testing) is to get a 375 H&H die to reset the case diameter (and hope the neck and shoulder of the Mashburn Case passes through the die without issue! If that all works - push the start of the shoulder far enough toward the base that the bolt will close and then fireform
Interesting slightly related thoughts
Belted magnums are an interesting beast. I originally thought I could adjust a lot of stuff making the cases by shortening the die, but forgot about the belt. I'm not a machinist and have no resources to do a good job cutting the die shorter or adding back the recess for the belt if I needed to in a die.
While all this effort is actually interesting - at the end I suspect I will have about the same invested in misc pieces for making the cases as I would have if I just bought a custom die for a few hundred dollars. But - they wanted fired cases. The wheel in the sky keeps turning.
