8mm Nambu Cartridge Conversion

[sheepherder]

One of my interests is cartridge conversions. This can be for pistols/rifles that are 'obsolete' and no one makes ammunition for them, or 'wildcats' that are not made at all except by enthusiasts. Conversions are not everyone's choice, but I find them fun & interesting.

Disclaimer: I am not responsible for any damage or injury incurred by these procedures. Wear safety glasses at all times; do not operate machinery while impaired. Approach any listed reloading data with caution. I am not soliciting work or services, and am not advertising anything for sale. There are many ways to do these cartridge conversion operations, this is just one of them. I encourage any interested parties to experiment and add your own experiences/conclusions.

8mm Nambu ammunition is hard to find. Making your own is the easiest way to get it. I decided that I needed a donor cartridge case at least 7/8" long, straight sided, and with a base diameter of ~.407".

My local gun shop has a box of 'range dug-ups' of all kinds. I took my vernier down and picked out some likely candidates. I'll start with the rimless first, since they're the 'easiest'.

Here is my assortment. All fired at least once; all old & dirty.

[first pic]

First order of business is de-priming them. I use the primer pocket to locate my cutting measurements from, so I use a 44 Magnum sizing die with a 22-250 de-capper. I don't want to stretch or squeeze the cases yet; this is the easiest way (with the assorted die sets I have) to de-prime the cases. The 44 sizing die does not come in contact with the case but holds the decapping pin in place.

[second pic]

Now I fit the case with a mandrel (just a drill bit that fits snugly in the case neck) and chuck it in the lathe. The tailstock has a live center and is set up so that pushing the case up against the tailstock/live center [set at 0] and then tightening the chuck will hold the case cutter right at 7/8".

[pics 3 & 4]

I break all sharp edges and de-burr in & out.

[pics 5 & 6]

Now we need to swage these cut down cases to size. These cases were all right at the ~.415" base measurement; I want .407" - .410". A member on another forum mentioned that by forcing rimless brass into a 30-30 Winchester sizing die, Nambu cases could be formed. The 30-30 die has the correct taper...IF you jam it in approximately 3/8" past the die rim. So to start, we run the cutdowns into the 30-30 sizing die as per normal fashion. Lubed generously.

[pic 7]

The next step is to remove the 30-30 die and invert it in my arbor press with a case inserted. I run the ram down until it is flush with the die.

[pics 8 & 9]

Now I position a plug to ram the case further into the 30-30 die, and press it in until it is flush. The case can just barely be seen after this operation.

[pic 10]

To get the swaged case out, I flip the die over and press it out with a 1/4" rod.

Here's a closeup of the die-to-press adapter, the plug, and the rod along with the 30-30 die and a compressed case. It is now ~.409" at the base.

[pic 11]

At this point I load all these cases into my tumbler and let it run overnight using crushed walnut shell media.

[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!

[singleshotman]

now that Starline now makes 41 colt brass, it can be used to make 8mm nambu brass. Neck to bottleneck and turn rim off and you are in business. I read this in an old Handloaders Digest, I forget the number, however. I do not own a nambu, but might buy one in the future.

[sheepherder]

Just reading on another forum about some Type 14 Nambus being able to use 357 Sig and 40 S&W cases without turning down the rim (or swaging them using the 30-30 Winchester sizing die)...My Type 14 will not take any rim over .417" and at that size it is tight...

Here is a comparison of the three...

[Sieger]

Hi,

Great post!!!

I also enjoy shooting the Japanese Nambu Type 14, and own several of them. Though not a "Luger", they are really of quite good design and are quite accurate when handloaded properly.

You should know that the .40 S&W has a rim that is too thick for the extractor of the Type 14. In fact, given the inferior metal these extractors are made of, they will often "spring" when fired with the .40 brass, leaving you no choice but to repair or replace them. As parts are generally not available, this can be troublesome.

I have some Federal brass originally made for the 8mm Nambu I obtained sometime in the early 80s. They were going to market with it, but the project was canceled, naturally. Fred Huntington ended up buying all of this brass and selling some of it to me, just by chance.

Just a few thoughts on a the very fine article above.


Sieger

[sheepherder]

Originally Posted by Sieger

You should know that the .40 S&W has a rim that is too thick for the extractor of the Type 14.

Thanks for reminding me!

I had noticed that it was thicker. On the cases that I had to cut an extractor groove, I also cut a bit off the rim. On the 40 S&W cases, I have manually stuck them in the bore and then racked the slide and they did flip out, so I left the rim alone.

I am not too far gone yet that I can't thin the rims on the 40 S&W cases. I see .042" rim thickness on the print. I'll check my 'test' cases and see how much needs to come off.

I recall seeing an ad or post somewhere about modified extractors for the Type 14 that would accept thick rims. Might have been Don Schlickman...I'll have to check his price list. I sent him a MO for a repro firing pin several weeks ago, maybe I can add an extractor to the buy...

Edit: Checking my 27 'test' 40 S&W cases, I'm seeing .052" rim thickness. The print I have shows .055". I'll rig up my parting tool bit and see what I can do to get it down to .042"...

Pic of the bag my gun club president dropped on me. I should be able to get several hundred out of these.

I forgot to mention that the conversion process does have a failure rate...About 1 in 15 or so is crushed or split during the neck forming operation...

[sheepherder]

I turned ~.010" off the rims on the 40 S&W conversion cases...I tried for .042"; I'm close to it...

[sheepherder]

Originally Posted by Sieger

I also enjoy shooting the Japanese Nambu Type 14, and own several of them. Though not a "Luger", they are really of quite good design and are quite accurate when handloaded properly.

Bob, I did a Search of this sub-forum to see if you had posted any of your Nambu handload data, and didn't find anything (only came up with six threads with Nambu in them, in this sub-forum)...

I still can't get any of the powder I'd like [4895, W231] but I have some Red Dot and Blue Dot left from before the freeze...

Do you have any loads for the 102gr FMJ Nambu bullet using Red or Blue Dot???

I was going to try a load for the 93gr 30 Cal Luger using Red Dot from a 1970 Lyman manual in my Nambu conversion cases...They list a low end of 3.9gr and max of 4.3...I may try 4.8gr to start, just a few to see if it cycles or if the primer flattens...Couldn't find any Red Dot data for the Nambu on the net...

[sheepherder]

Bob/Sieger's comment about the rim got me thinking, and I ran my 40 S&W cases on the lathe and turned the rim down to approx .042"...(from approx .052")...But when I ran my mixed bag of other brands/calibers, there was some variation, as much as +/-.004"...

I was using a live center and locating off the edge of the primer pocket...But they weren't all the same...Some had a tapered edge, some not so much...So my 'zero' wasn't quite right...

So to make a better 'zero' I turned a piece of Nylon down to fit in my tailstock Jacobs chuck with a small primer 'nub' [.175"] on one end and a large primer 'nub' [.210"] on the other end...

Now I clamp this mandrel in the Jacobs chuck, set it at zero, push a case up onto the mandrel and then clamp the neck in the lathe chuck...Case is centered and rim base is up against the mandrel...And now I can cut my rim...

Works much better! Variation is cut down to approx +/-.001"...

I would rather use Teflon for my mandrel...But I'm all out...

[Sieger]

Originally Posted by sheepherder

Bob, I did a Search of this sub-forum to see if you had posted any of your Nambu handload data, and didn't find anything (only came up with six threads with Nambu in them, in this sub-forum)...

I still can't get any of the powder I'd like [4895, W231] but I have some Red Dot and Blue Dot left from before the freeze...

Do you have any loads for the 102gr FMJ Nambu bullet using Red or Blue Dot???

I was going to try a load for the 93gr 30 Cal Luger using Red Dot from a 1970 Lyman manual in my Nambu conversion cases...They list a low end of 3.9gr and max of 4.3...I may try 4.8gr to start, just a few to see if it cycles or if the primer flattens...Couldn't find any Red Dot data for the Nambu on the net...

I haven't done as extensive work with the 8mm Nambu as I have the 9mm Luger, but a good powder for me was Unique with the Nambu. When I find the time, ha!, I'd like to try my favorite Power Pistol and SR 4756 powders with the Nambu, as I think they would do quite will in this case.

Also, start low with your above load, say 3.5 grains of Red Dot or so, and work up until the action will just stay open. By doing so, you will have established the bottom load of your relevant range. Carefully work up one tenth of a grain from there and note accuracy. Also, a hot load is not usually an accurate one, in most cases.

Go ahead and try the same 3.5 gr starting load with your Blue Dot powder, working up until the hold open catches to find the bottom of your range as note above. Blue Dot for me has always been a flame thrower, but with bottle neck cases it may perform a bit better.

Sieger

[sheepherder]

I ran across this powder chart for the 8mm Nambu in another forum...It was supposedly copied from the Midway site some years back...

I haven't seen anything like that on Midway now...

[rhuff]

I am not too sure how well the Blue Dot powder will work for you, but it is sure worth a try since you have some in hand. Blue Dot really shines when you load it up near or at max loadings. It is a magnum powder and likes to be loaded hot. I use Blue Dot in 357 Sig brass, and it produces superb velocity and accuracy. The only down side(to some at least) is the flash/bang that it produces. It is quite impressive!!!

[sheepherder]

Originally Posted by rhuff

Blue Dot...is a magnum powder and likes to be loaded hot.

I'm going to use the Red Dot first. Right now I'm trying to figure out this 3-die set, as it has no instructions and there is one die that I'm puzzled by...

[rhuff]

Was it originally a 3 die set, or has someone added a die, and forgot to tell you?? With your experience in things like making modified/reformed brass and reloading, I can't imagine what kind of a die has you stumped!! You have definitely peaked my attention.

[sheepherder]

Originally Posted by rhuff

Was it originally a 3 die set, or has someone added a die, and forgot to tell you?? With your experience in things like making modified/reformed brass and reloading, I can't imagine what kind of a die has you stumped!! You have definitely peaked my attention.

Right. OK, here goes:

RCBS die set as shown in post #5 above. First pic below shows all three dies in the set I received, disassembled. They are marked as follows, in order as shown -

[first die] RCBS 8mm Nambu Size # 170 on die body

[second die] RCBS 8mm N on ram; no die marking

[third die] RCBS 8mm Nambu Seat #370 on die body

Second pic below shows just the middle die, with dimensions as shown.

Here's the problem...The ram in the middle [expander?] die expands the cases so that a .321" bullet falls right in. Very good fit; I can feel the air rushing out the primer hole. The ram also has a taper leading to another expanding size which is a sliding fit in a 38 Special/357 Magnum case but does nothing to an 8mm Nambu case unless you push it down to the beginning of the taper, when it will give the case mouth a very slight bell...But if you expand the entire neck, there is nothing to hold a .321" bullet in place to crimp it...It falls inside...

Pretty much every book I have [including COTW] shows the Nambu bullet as .320", which is even worse...

I first thought the middle die was the accessory die for expanding the 38 Special case, but the entire ram slides right through & bottoms out...Without touching the 38 Spl case...So it's not the expander for making 8mm out of 38 Special...

I can just push the middle die ram down a slight bit in my formed cases, but then I get no bell...

So...WTF is this die for???

[sheepherder]

Originally Posted by Rick W.

One would suspect that the first die oversizes the case, then the middle die first portion sizes the interior of the neck, then the option to bell is available.

That is what most of their 3-die sets do. This one is different...

Brass thickness would not affect the expander; it's going to expand it to .321" ID no matter what the case wall is...

RCBS may have made these dies for a larger bullet; say .322"-.323"...But besides Lebel & Nagant there haven't been any .322"-.323" 8mm pistol cartridges...According to COTW...

Could entertain taking the inside sizing plug, the first area, and knock it down a little for grip on the bullet in hand.

I expect I will end up making another expander ram for my home made die body and try to get it right around .320"...Make the first 3/16"-1/4" @ .320" and then a quick taper for a slight bell on the case mouth...

I'll try emailing RCBS and ask if they still have instructions for this die set and maybe find out what they intended...

Edit: HMOCC recommends using "Lyman bullet #32362 sized .321"...Maybe RCBS made their die set for this one bullet??? Maybe it was the only one available back when the 36404 die set was offered???

Edit2: This site shows the Lyman 32362 as a .325" cast bullet...

http://www.darkcanyon.net/lymancastbulletinfo.html

I tried the Lyman site; that number doesn't show anything...

Edit: Emails sent to both RCBS and Lyman. Waiting for replies.

I'm getting too old for this stuff anyway. I should be looking into shuffleboarding...Bingo...TV...Pottery making...Stuff that people my age do...

[Zorba]

Originally Posted by sheepherder

I'm getting too old for this stuff anyway. I should be looking into shuffleboarding...Bingo...TV...Pottery making...Stuff that people my age do...

How about Belly Dancing?