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.