350 Mashburn Short Magnum - seeking dies!

[MGMLVKS]

Well nothing is easy! Appears that forming the taper just pushes brass down. One of these cartridges is right at 2.05" for start of shoulder, the other is quite a bit deeper. Neither will chamber. Ughh. The diameter of case should be 0.480/0.485 but formed cartridges are 0.490-0.495 where the brass has moved. Bought a set of 375 H&H Mag to re-profile the case. Never thought it would be easy . . . .

 

[AlNyhus]

Just a question....how did they make cases for this chambering 'back in the day' ?

 

[MGMLVKS]

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.

 

[AlNyhus]

Why not move the belt area of a 358 Norma full length die forward and then shorten the back end of the die?

 

[MGMLVKS]

Why not move the belt area of a 358 Norma full length die forward and then shorten the back end of the die?

Another good question! This was my original thought - that by shortening the die I could sort of force the neck down by a few hundredths to shorten the distance of the taper to the neck and not really disturb the basic case dimensions. But I forgot about the belt. I would have to remove the recess for the belt inside the die - which is really a machine shop operation! That is not me and my "people" that used to do that sort of thing have all moved up to the next level, and the shops around here are not really interested in this type of fine detail work.

I hope to be back on this in earnest next week. In the meantime I'll try to get the case diameter back in compliance with the 375 H&H die, play with some candle soot and maybe - just maybe - I'll get something the bolt can close on!

 

[AlNyhus]

You could also just cut the back end of a belted die off above the belt area.This would keep the shoulder and most of the case body supported when the shoulder is moved back...that would keep the body/shoulder junction from expanding out.

If the Cerrosafe numbers are right, the chamber shoulder diameter is about .009-.010  smaller than some of the dies you're thinking of using. What shoulder angle does the Cerrosafe show it to be?

 

[Sniperhandle]

I would take your Cerrosafe sample that you had casted, And measure it against what you have formed.

The 350 Ackley must have less case taper than the Mashburn.

Once you get fired cases you will be home free and can use the stuff you have gathered.

 

[MGMLVKS]

I would take your Cerrosafe sample that you had casted, And measure it against what you have formed. The 350 Ackley must have less case taper than the Mashburn. Once you get fired cases you will be home free and can use the stuff you have gathered.

Thanks - It turns out it was not the cases or the dimensions - it was my brain not fully engaging! It was driving me nuts! Ultimately - I may not need the step to run the cases back through my new/used 375 H&H die to reset dimensions in the cartridge!

Story Follows: I had made two cases from 358 Norma magnum brass. The bolt would not even think about closing on, and both were "really close" to the chamber cast dimensions as well as the dimensions in Donnelly. One was "at" the shoulder length from the 35 Ackley die (same dimension - 2.05"), the other was pushed down far enough to get away from any shoulder/taper dimension problems. So I figured I had abused the cases enough that running them through a 375 H&H Magnum Die (without the expander) ought to sort out any dimensional indignities and allow me to sort of start over. Dies came in over the weekend and I did that this evening. All I really needed to do after that was trim cartridge length a few thou, and EVERYTHING was now at or below the numbers I had for the chamber!!

Still - ZERO Interest in the bolt closing. OUCH!!. Resisted the temptation to quit, get an adult beverage and watch sports for the rest of the evening and then put my engineer brain to work on it. I had taken a M1903 drill rifle completely apart for another project two years ago, like 99.9% apart. Great Fun - learned a lot!!. That meant I knew how all the pieces fit and worked together.

Step by step I went through the process of what could possibly be happening. Obstructions, misreading the micrometer, misreading the dimension diagrams, missing pieces, etc., no, no and NO. Finally - and I am apparently an idiot - realized it was the extractor that was not going over the rim and was just sitting there! No wonder it would not close!!

Took the two cases I had and placed them lovingly inside the extractor claw, and the one that was pushed down closed without any issue or annoyance. The one that is "at" the dimensions closed just fine with a little extra pressure the last little bit on the handle

I AM VERY HAPPY!

Now to create a third cartridge with what I think is the right process (with notes!), then a bit of fire-forming, and maybe - just maybe - make it happen!

 

[Sniperhandle]

Thanks - It turns out it was not the cases or the dimensions - it was my brain not fully engaging! It was driving me nuts! Ultimately - I may not need the step to run the cases back through my new/used 375 H&H die to reset dimensions in the cartridge!

Story Follows: I had made two cases from 358 Norma magnum brass. The bolt would not even think about closing on, and both were "really close" to the chamber cast dimensions as well as the dimensions in Donnelly. One was "at" the shoulder length from the 35 Ackley die (same dimension - 2.05"), the other was pushed down far enough to get away from any shoulder/taper dimension problems. So I figured I had abused the cases enough that running them through a 375 H&H Magnum Die (without the expander) ought to sort out any dimensional indignities and allow me to sort of start over. Dies came in over the weekend and I did that this evening. All I really needed to do after that was trim cartridge length a few thou, and EVERYTHING was now at or below the numbers I had for the chamber!!

Still - ZERO Interest in the bolt closing. OUCH!!. Resisted the temptation to quit, get an adult beverage and watch sports for the rest of the evening and then put my engineer brain to work on it. I had taken a M1903 drill rifle completely apart for another project two years ago, like 99.9% apart. Great Fun - learned a lot!!. That meant I knew how all the pieces fit and worked together.

Step by step I went through the process of what could possibly be happening. Obstructions, misreading the micrometer, misreading the dimension diagrams, missing pieces, etc., no, no and NO. Finally - and I am apparently an idiot - realized it was the extractor that was not going over the rim and was just sitting there! No wonder it would not close!!

Took the two cases I had and placed them lovingly inside the extractor claw, and the one that was pushed down closed without any issue or annoyance. The one that is "at" the dimensions closed just fine with a little extra pressure the last little bit on the handle

I AM VERY HAPPY!

Now to create a third cartridge with what I think is the right process (with notes!), then a bit of fire-forming, and maybe - just maybe - make it happen!

I hadn't thought of you possibly not knowing that the 1903 Springfield is just like a Mauser in that you need to feed it from the Magazine as it is also CRF.

 

[MGMLVKS]

I hadn't thought of you possibly not knowing that the 1903 Springfield is just like a Mauser in that you need to feed it from the Magazine as it is also CRF.

Me neither! My M1903 experience is shaped by the guidance that the soldiers were using single shot until ordered to flip the lever to use the magazine. Well - great plan that did not carry over well to the great Sporterization and Wildcat experiments post war. I've learned something today - and that makes me quite happy!

 

[AlNyhus]

The best fix is the easiest one!

 

[MGMLVKS]

GREAT Progress after the GREAT Enlightenment!

350 Mashburn Short Magnum
Cartridge Creation (Work in Progress!)
McDonald 20-OCT-2025

Completed Steps as of 20-OCT-2025:

• Find 358 Norma Brass, preferably NEW! Although the 350 Mashburn Short Magnum and many other similar magnums are similarly based off of 300 H&H and 375 H&H Magnum why not start with one that is REALLY CLOSE in dimensions to the final product?!
• Use a 375 H&H Magnum Full-Length Die to Resize the Norma 358 Magnum brass after removing the Expander Assembly.
• Use 35 Ackley Magnum die to set the shoulder dimension from the base of the case. Both cartridges have that distance as 2.05”! In my situation – the bottom of the 35 Ackley Magnum die had (unbeknownst to me) been cut off – which means you have to adjust the die downward until you get the start of a shoulder at the right place. As my die was “adjustable” with the base removed, I went with ~2.04” to allow some clearance with the shoulder angle at the neck.
• Check case for function in the rifle. All three cases cycled through the rifle, having a firm but not “hard” closure needed on the bolt.
• Case length came out at 2.53’, but needs to be 2.475. I left this alone for now as the excess brass may be pulled down if the Mashburn 30 Degree shoulder fire-forms properly.

Next Steps:

• Do some fire-forming on these three cases with the “Cream of Wheat” technique. Sources vary somewhat in how many cycles of fire-forming are necessary – I figure at least two.
• Trim cases to proper dimension of 2.475
• Load the three cases with the lowest recommended power/pressure and test fire
• If successful – do a bit of bore-sighting
• Form the remaining 358 Norma Mag cases into 350 Mashburn Short Magnum cases.
• Take a trip to the range!

 

[Sniperhandle]

12 grains of Unique will be just about right for that size of case topped with cream of wheat, Corn meal, Grits, Or Malto meal work very well. Take a candle and light it away from your loading supplies and let it burn long enough to have a well of liquid wax, Pour it into a jar lid and let it cool. Then hold the jar lid upside down and push the necks of your just filled cases with the cream of wheat and twist when it bottoms out and pull it it out. Repeat for the rest of them.

Use large pistol primers.

This will form a seal that makes it easy to cycle through the action without spilling COW.

 

[MGMLVKS]

12 grains of Unique will be just about right for that size of case topped with cream of wheat, Corn meal, Grits, Or Malto meal work very well. . . . Use large pistol primers.

Picked up 1lb Unique and 100LP Primers. My lucky day as been off the shelf for over 2 years!

 

[Sniperhandle]

I forgot to tell you, The shoulders will form pretty well, But will have a slight radius at the edge of the shoulder, It won't fully straighten that edge up until you load and fire your first projectiles.

I have tried heavier charges of unique and it just never will get that radius out until you shoot projectiles.

Clean the barrel well to remove any residue from fire forming.

 

[MGMLVKS]

I forgot to tell you, The shoulders will form pretty well, But will have a slight radius at the edge of the shoulder, It won't fully straighten that edge up until you load and fire your first projectiles.

I have tried heavier charges of unique and it just never will get that radius out until you shoot projectiles.

Clean the barrel well to remove any residue from fire forming.

Thanks! I probably need to look at my stockpile of powder and Primers from years back to see what is needed to move forward. I have loading data from "back in the day" references as well as some more modern data from the internet which must be right! Ill try and lay it out for rational decision making. I have both 200 and 250gr projectiles. Need to spend some time understanding the scope as well - been off/on a few times - so dust off my trajectory tables and associated brain cells. My fave public range near St. Joseph MO was closed this year due to continual vandalism so will prob do fireforming at girlfriends 20ac at Bonner Springs and not sure on final product testing. Has been an absolutely fascinating journey and education so far!