8mm Nambu Cartridge Conversion

[sheepherder]

There are several sites with mention of using 38 Special cases to make 8mm Nambu cartridges. RCBS even makes a conversion die set. I decided to make some of my own 8mm cases from 38 Special.

Using once fired cases, I de-primed them using my 44 Special sizing die with the 22-250 decapping pin. The rim needs to be turned down to ~.415", so I mount a case on a 9mm drill bit and clamp it in my lathe chuck. Using a parting tool I turn the 38 Special case rim down to ~.415"

[pic 12]

Next step is to cut the extractor groove, again using a parting tool, but smaller. I cut it to .357" dia.

[pic 13]

After grooving, I cut a chamfer on the forward edge of the extractor groove, using the wider parting tool again.

[pic 14]

After the chamfer is cut and before releasing the case from the chuck, I run a file against the outer edge of the rim to facilitate the extractor engaging the case when feeding.

Cases are now ready for forming.

[sheepherder]

I also made up some 38 Special cases without cutting the extractor groove. But I needed to expand the cases, as the OD was [IMHO] excessive for the 8mm chamber - Something like .030" smaller. I've been told that unexpanded 38 Special case used for 8mm Nambu could/would split when fired. To counter this I made up my own 'expanding die' out of 1" steel rod.

[pic 1]

Turned down to ~ 7/8" diameter. The die body was also drilled & internally threaded to 1/2" x 20tpi for my 'ram'. The die body was also drilled for 3/4 of its length to 27/64" to contain cases expansion.

[pic 2]

Threading the die body OD to 7/8" x 14tpi.

[pic 3 and 4]

Turning the 'ram' out of a 1/2" x 4" SHCS.

[pic 5]

Expanding die in RCBS press, cranking out expanded 38 Special cases.

[pic 6]

Close-up of expanding die.

[pic 7]

Some expanded cases.

[pic 8]

One of our members mentioned having reloaded 8mm Nambu ammunition that was made from 40 S&W brass. I was able to filch some once-fired 40 S&W cases out of the brass barrel at my local club for conversion.

Using the fired cases, I de-primed them using my 44 Special sizing die with the 22-250 decapping pin. I left the case rim and extractor grrove alone.

I've read of extraction problems with the 40 S&W case conversions. The 40 S&W case is oversize for the 8mm Nambu specs, so I swage these cases. These cases were all right at the ~.415" base measurement; I want .407" - .410". I use the 30-30 Winchester sizing die again to resize the cases, same procedure as described previously for oversize cases.

My 40 S&W cases are now ready for neck forming.

[sheepherder]

Here are the cases that have been swaged or expanded. They are -

38 Special
375 Winchester & 30-30 Winchester
25 Remington & 32 Remington
40 S&W

These are now all straight-walled cases. The next step is to form the case necks in the RCBS 8mm Nambu sizing die.

[sheepherder]

Forming the neck.

RCBS 8mm Nambu 3-die reloading set. This is an older set, and the seller included an RCBS #25 shell holder. Nothing special here; lube the case liberally and run it through the RCBS 8mm Nambu sizing die with de-priming pin removed.

[Pic 1].

40 S&W in press.

[pic 2]

38 Special in press.

[Pic 3].

Trimming OAL to ~.862".

[Pic 4]

The various cases neck-formed but not yet trimmed.

[pic 5]

An experiment using a suggestion by a member in another thread (which I can't find now; sorry!), to power trim the cases.

[pic 6].

Finished 40 S&W case converted to 8mm Nambu and ready to reload.

[pic 7]


Notes

- The 375 and 30-30 Winchester cases were the most difficult to form & neck
- The 375 and 30-30 cases need the primer pocket reamed; the rest don't
- Winchester brass is softer than Remington, R-P, and military
- The rifle brass has thicker case walls, thicker base, and large primer
- 38 ACP/38 Super cases could also be expanded to form 8mm Nambu in the same way as the 38 Special. No groove need be cut and the rim need not be trimmed. I did not have any so there are no pics/comments.
- I test fit about 1 in 10 in my Type 14 Nambu; they all fit; bolt closes; extractor engages.

Conclusions

While many cartridges can be used to convert to 8mm Nambu, I found the 40 S&W to be the easiest.

The RCBS 36404 3-die set seems to be the best for forming & reloading.

In a surprise move, the president of our gun club presented me with a bag of 40 S&W once-fired brass last night. What a guy!