10mm Auto Rim?

[Bullwolf]

I was looking at a 401 Herter's Powermag cartridge, and then I started thinking. I wonder why there is no 10mm Auto Rim.

I noticed that 401 Powermag was pretty close already, and it can be formed from 41 Magnum brass, or even 30-30 Winchester brass. (Though if made form 30-30 brass it has slightly less internal case capacity)

So why couldn't I make some brass like this to fit in my Smith 610 revolver and quit being married to moon clips?



I started out looking at cartridge specs for the 401 Powermag, 41 Magnum, 30-30 Winchester, and the 10mm automatic. Soon I was measuring cases, trying stuff, and the wheels started turning.





It looks like quite a squeeze to get 41 Magnum down (about .009) but the rim and internal case capacity look promising. I have heard of folks doing this with no shell holder, or a shaved shell holder to size 41 mag all the way down to the rim, and then using a hammer and punch to get the 41 Magnum brass out of a full length size die to form 401 Powermag.

Using 30-30 Winchester brass seemed easier than sizing 41 mag brass down that far somehow. I also have a lot more 30-30 brass than 41 Magnum, so that helped sway the decision. I snagged a 30-30 case, shell holder, and with my trusty RCBS case trimmer, I shortened a 30-30 case down to .992

I de-burred the case and did some light inside neck reaming. I probably should have annealed before case flaring and seating as well. I did some revolver cylinder mock ups using the new case.

The first little speed bump I ran into was putting a large pistol primer into a case that already uses large rifle primers.

I removed the spent 30-30 large rifle primer, pulled the anvil out of the cup, and knocked the firing pin dent out of the primer with a small punch.

I reseated this dummy anvil-less primer in the case, and tested my firing pin hit for strike and protrusion. The 610 put a dent in the deactivated rifle primer, so the whole thing seemed feasible so far.

Then, I seated a Win large pistol primer in the 30-30 case... Oops forgot large rifle primers are a tad taller than large pistol primers. I noticed .120 and .130 difference between the two. So a large pistol primer seats in about .010 too deep in the rifle case. I guess that's always better than sticking out, but I didn't like it. It seemed like I was already on the minus side of things for firing pin protrusion from not using moon clips.

So I pressed the primer out, and made a very small thin copper shim washer. I pressed the shim into the primer pocket and reseated a pistol primer flush. Perhaps this is why 41 Magnum PISTOL brass was a more popular choice as a parent case for the 401 Powermag.



Well anyways, I got this far with it.

In the pictures the case is not flared yet, and I just have an upside down 10mm FMJ stuck in the case for perspective. (not seated or crimped) It seems doable so far.

I still need to measure the internal capacity of the case to get an idea where I will be at for a load. Assuming the new case has the same capacity at least as 40 s&w and not less, I was planning on starting with a light 40 s&w load, and seating to long 10mm OAL's to keep pressure on the safe side of things.

I was wondering if anyone else has ever tried something like this? Since I had never seen 10mm Auto Rim brass before.



- Bullwolf

[ReloaderFred]

I thought about it once for my 610, but instead bought the Mooner and Demooner tools to make loading the moon clips a cinch. In the long run, it was much easier than messing with all that work to make a few cases.

Hope this helps.

Fred

[rintinglen]

Get a 10 mm case and your case and weigh them. Take your case and fill it with water, then dump the water, into the 10 mm case and weigh that. Then top off the 10 MM case and weigh it again. The difference will be proportional to the change in internal volume and will give you a guideline on how much of a charge reduction you'll need to make to keep pressures similar.
To get how this works consider the below hypothetical case.
Say your empty 10mm case weighed 135 grains, and it weighed 190 grains with the water from the altered 30-30 case. When you top it off, it now weighs 195 grains. The altered case will have 55/60ths or roughly 92 % the volume of the un-altered case. Decreasing the load by 10 % just to be safe and fire it over a chronograph. the velocity should be similar to what was produced before, perhaps a smidge lower. Your actual numbers will be different, but the process is the same.

[gwpercle]

I say go for it....that's why we reload!

[Dale53]

No one has mentioned the headspace issue here. .45 Auto rim cases have the unusually thick rim to make up the difference between using a rimless with moonclips vs. using "normal" rim thickness (such as the .45 Colt). Your parent case (the .30/30) has a normal rim thickness. The headspace in your revolver will be excessive.

Not a good plan...

FWIW
Dale53

[Bullwolf]

No one has mentioned the headspace issue here. .45 Auto rim cases have the unusually thick rim to make up the difference between using a rimless with moonclips vs. using "normal" rim thickness (such as the .45 Colt). Your parent case (the .30/30) has a normal rim thickness. The headspace in your revolver will be excessive.

Not a good plan...

FWIW
Dale53

Well the idea was still in the evaluation stage. Thank you for the head space warning.

For comparison purposes on my 610 I can slide a .045 feeler gauge between the firing pin bushing and the cartridge using the trimmed 30-30 case. I can only slide a .015 feeler behind 10mm on a plastic (my fattest) full moon clip.

I am no revolver smith, but I thought .060 to be in spec, and .074 or larger to be excessive head space for a typical 38 special revolver. No clue how it works with a cartridge set up to head space on moon clips.

Looks like it was a good idea asking if anyone here has tried to make 10mm Auto Rim before.



- Bullwolf

[Treeman]

Since you are dealing with a chambering that headspaces on the case mouth, rim thickness need not have any bearing. Trim to the proper length and headspace off the mouth and the rim can simply hangout back there to facilitate ejection.

[ReloaderFred]

In the 610 revolver, the headspacing is changed from the case mouth to the moon clip, which is why it's possible to shoot .40 S&W through them. Many people are shooting .40 S&W through their 610's because it's cheaper and more readily available. If the case headspaced off the case mouth, this wouldn't be possible. The moon clip makes it more like the .357 Magnum/.38 Special situation, where the headspace is at the rear of the cylinder, rather than the case mouth.

Hope this helps.

Fred

[Treeman]

Fred, I haven't handled a 610 but I would have expected S&W to cut the chambers properly for the 10mm. Using moon clips allows a shorter cartridge such as the 40S&W to headspace off the clip but there should be a shoulder for the case itself to headspace on IF the case is 10mm auto length.