Modifying mold for 8mm Nambu

[RacinJacin]

New member figured I ought to jump right in... found myself in need of a 8mm Nambu bullet mold. Quickly realized there's no way I'm paying for one. I did find a cheap RCBS 30-115 mold.
My plan is to take a .320-ish reamer and grind a short pilot on it. Then use an old Bridgeport to modify the mold.
A couple questions come to mind:
1. Exactly what reamer size should I shoot for if I wanted a .323" bullet. I do PC and size afterwards so I'm probably not too worried there, but would certainly appreciate some guidance from those that have "been there, done that".
2. How critical is the reamer RPM? I would tend to run it on the slower side, but again input would be great.
3. Should I even worry about a crimp groove? I'm leaning towards not as this round is pretty anemic from what I understand.

Any input would be great. And just to be clear I understand I'd be better off to send this out to someone more familiar, but I'm not wired that way.
I didn't pay much for this mold so I don't mind taking a risk on it. And yes I do realize this would end up a tad heavy. My thought there would be to cast up a bullet, and then machine it down 'til it weighs what I want. Then go back and skim cut that amount off the top of the mold.

With any luck this seems to be a cheap way to start casting the 8mm boolits I need.

Thoughts and suggestions are appreciated.



Jacin

[RacinJacin]

My other thought is that I ought to start with a 32acp mold as that seems to be the popular path as far as modifying molds for 8mm Nambu. In looking that over I'm not sure why that's the easier path. Not sure if its worth even considering that or just stick to what I already have. Inquiring minds.

Jacin

[BP Dave]

I'm not a machinist, so can't offer much help but am interested in the project. I have made a couple of bumping dies out of old bolts using D-reamers (for bumping .30 carbine jacketed bullets up to .322), but my mold modifications have been limited to increasing band diameters a few 1000ths.

Are you talking about reaming the mold? Or are you making a cherry from your reamer? If you're reaming from the bottom, of the mold, I don't understand the need for a pilot on the reamer and also don't know how you would be able to create a crimp groove.

Perhaps a drawing of what you propose would help.

And yes, 8mm Nambu is pretty anemic. If your reloads aren't, you're probably doing something wrong.

--DJ

[RacinJacin]

Yes, in the simplest approach I would just ream it and be done. That mold has lube grooves which I wouldn't need since I plan on powder coating anyways. The pilot is overkill admittedly, just a thought to aid in getting the concentricity as dead on as possible. The only (stretch I am sure) reason would be if the original "hole" isn't perfectly perpendicular to the part the pilot would to a degree guide it in. Probably overthinking that.
As for the crimp groove that is another can of worms since I can't easily add material to the mold, but I could modify a lube groove. I'm not considering a cherry here as I just was looking for the path of least resistance. I think as anemic as the round is, a crimp groove is really way over the top. I haven't done the math but I doubt I could get the existing geometry to work anyways.
The more I contemplate this I think just removing the lube grooves with a reamer is more than enough to get where I want to go here. Edit: OK I am slowly getting my head on straight. I read at one point reworking 38spl brass and I guess that made me think I needed a crimp groove. Duh! I do not. Problem solved there. So I'm left with my shrinkage question...using some Kentucky windage I calculated about .002" shrinkage. Which makes me think that using a .323 reamer is probably gonna be OK if not just a wee bit small. I could always modify the reamer to cut a thou. or so oversize. Looks like I will have to go thru the reamer drawer and see what I can find.
Any thoughts on sizing/shrinkage would be welcomed! Thanks


Jacin

[RacinJacin]

Update: Didn't have a reamer on hand...so I opted to bore it out. I bored it to .325" figuring on .002" for shrinkage. I bored it .380" deep which was purposefully conservative. I did this just in case I decided to blend the transition. After looking at it I won't bother. With my range scrap lead I was now casting at 119.2 grains. Did some math and figured I needed to remove .082" to get the weight to my target of 100gr. To verify that I chucked up a bullet in the lathe and took .082"off. It now weighed 100.5 grains. Close enough! Skimmed .082" off the mold and also had to go back and deepen the pivot screw counterbore since barely any was left now.
Original bullet was about .697" long 119.2 grains
Final bullet casts to .615" long 101.4 grains .324" dia.
Disclaimer: My measurements were done with calipers and as such are subject to a thou. or so deviation.
I didn't yet have the time to run full casting session, so my mold might not have been to the optimal temp. Nor was my lead "mix" any particular ratio. Having said all that I am claiming this is thus far a success!
One with great care could open these up on a drill press, but I suppose that's mute as you'll eventually need a mill to shorten the mold. In any event this has proven a cheap path to getting some 8mm Nambu boolits!
I hope this helps someone! So far I'm tickled with it.

Jacin