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Thread: Forming 8mm Nambu

  1. #1
    Boolit Buddy

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    Forming 8mm Nambu

    Hello,

    This is my first foray into case reforming. I have the manual of cartridge conversions and it says to start with a Rem 30, cut, size, ream the neck, and resize. Iíve also read how members here use 40 S&W pushed through a 30-30 die and then through the nambu sizing die. I have two questions I would be grateful for advice on:

    Did either of these approaches work better for the extractor/give less on the extractor?

    If I neck ream the 30 formed cartridge, do I simply need an 8mm reamer pilot for my RCBS trimmer - would that be the right size? Or should I order a custom diameter neck reamer?

    Thank you

  2. #2
    Boolit Grand Master
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    Never done the 8mm Nambu, but I do have some thoughts on it.

    Almost all of the books were written decades ago when you could still get .30 Remington cases, but before the 10mm or .40 S&W came out.

    From posts I have read in the last few years, the easiest way is to start with .357 Sig. Which is basically a necked down .40 S&W.

    As for reaming the necks after forming, I have no earthly idea if it is needed or not.

    Robert

  3. #3
    Boolit Buddy

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    Quote Originally Posted by Mk42gunner View Post
    Never done the 8mm Nambu, but I do have some thoughts on it.

    Almost all of the books were written decades ago when you could still get .30 Remington cases, but before the 10mm or .40 S&W came out.

    From posts I have read in the last few years, the easiest way is to start with .357 Sig. Which is basically a necked down .40 S&W.

    As for reaming the necks after forming, I have no earthly idea if it is needed or not.

    Robert
    Oh I think that makes some sense!
    Thank you!

  4. #4
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    Do a search here, there are several posts on forming brass for 8 Nambu. I have formed it from both 357 Sig and 40 S&W. The 357 Sig would seem to be easier, but they both formed pretty much the same. One pass through the sizing die.

    I made a push through die to reduce the 40/357 Sig case diameter to Nambu size. It takes a stout press and of course case lube.

  5. #5
    Boolit Bub
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    As others suggest, .357 SIG is probably the easiest to reform. You will likely need to swage the base, which, as Garandsrus states, takes some heavy pressing. I suggest you use a push-through die or the .30-30 for that. You can use the Nambu die to form the upper part of the .357 sig to 8mm, and you can use it to size cases after they are fully formed, but you probably don't want to risk cracking that expensive die by using it to swage the head.

    I am probably too extravagant--I made a push-through die to swage the heads using a 3-ton arbor press (it was cheap at the time) and then clean up and thin the rim using a lathe. The neck is pretty forgiving on a Nambu, but I still find that I need to lightly ream the neck to get .3225 bullets loaded for my T14. You might avoid that with a .320 bullet that some folks use.

    But to reiterate, you are courting trouble if you use the Nambu sizer to try to swage the case head.

    Good luck.

  6. #6
    Boolit Master
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    My brief foray into the Nambu was forming it from 357 Sig brass. I did find it necessary to ream the neck and used a copper reaming tool made for plumbing. Mine has pretty forgiving tolerances, so maybe it's not the best for everyone. I learned to do the conversion by searching on this forum.


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  7. #7
    Boolit Master



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    Obtain 40 S&W cases. Get a Winchester 30-30 sizing die and remove the decap stem.....place the die in the press. Do NOT use a shell holder in the ram. Instead, place a piece of flat steel on top of the ram, lube the case with sizing wax and push the case ALL THE WAY HOME. Lower the ram and use a piece of 1/4" brass rod to drive the case from the die.

    What you just did: The case diameter of the 8mm Nambu happens to be very close to the diameter of the 30-30. By pushing the case all the way in, you are also reducing the base and rim in one shot.

    Okay, now you have a straight walled, ex .40 S&W case. Place an 8mm Nambu die and shell holder in your press, lube the case again and run it into the die in progressive stages (this allows the shoulder to form easier) until a completed case emerges. Load with .321 boolits over some Unique and enjoy.

  8. #8
    Boolit Buddy

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    Quote Originally Posted by 3006guns View Post
    Obtain 40 S&W cases. Get a Winchester 30-30 sizing die and remove the decap stem.....place the die in the press. Do NOT use a shell holder in the ram. Instead, place a piece of flat steel on top of the ram, lube the case with sizing wax and push the case ALL THE WAY HOME. Lower the ram and use a piece of 1/4" brass rod to drive the case from the die.

    What you just did: The case diameter of the 8mm Nambu happens to be very close to the diameter of the 30-30. By pushing the case all the way in, you are also reducing the base and rim in one shot.

    Okay, now you have a straight walled, ex .40 S&W case. Place an 8mm Nambu die and shell holder in your press, lube the case again and run it into the die in progressive stages (this allows the shoulder to form easier) until a completed case emerges. Load with .321 boolits over some Unique and enjoy.
    30-30 just bought on eBay. Thank you!

  9. #9
    Boolit Master
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    Quote Originally Posted by 3006guns View Post
    Obtain 40 S&W cases. Get a Winchester 30-30 sizing die and remove the decap stem.....place the die in the press. Do NOT use a shell holder in the ram. Instead, place a piece of flat steel on top of the ram, lube the case with sizing wax and push the case ALL THE WAY HOME. Lower the ram and use a piece of 1/4" brass rod to drive the case from the die.

    What you just did: The case diameter of the 8mm Nambu happens to be very close to the diameter of the 30-30. By pushing the case all the way in, you are also reducing the base and rim in one shot.

    Okay, now you have a straight walled, ex .40 S&W case. Place an 8mm Nambu die and shell holder in your press, lube the case again and run it into the die in progressive stages (this allows the shoulder to form easier) until a completed case emerges. Load with .321 boolits over some Unique and enjoy.
    Excellent description! I use 32 Special dies in place of yours though.
    How did you get away without inside reaming the necks? Are you thinning the wall enough with the 30 caliber die that it's not needed? Is it simpler with the 40 cases vs. 357 Sig?


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  10. #10
    Boolit Master
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    I started trying to make 8mm Nambu with 40 S&W cases, then switched over to 357 Sig brass. My plan was to use a 30-30 sizing die (expander and decapper removed) to start this process but found that this die did not let me size down the bottom of the cases all the way to the extractor rim when using a piece of flat steel on top of the press ram. It seems that the 30-30 sizing die is slightly open at the bottom of the die. I visited a friendly machine shop and brought a shell holder with me as a partial sample of what I needed. My request was for a shell pusher that would snap into the press ram like a shell holder, yet the area above the ram must be small enough to fully enter the 30-30 sizing die, plus long enough to push all of the Sig case into the die. The final product was very satisfactory and sizes all of the cases at the rim area. Next stop was to form the top of the cases in a Nambu die, trimming to length and de-burring. No neck reaming was required. Please remember to lube each case that will be sized, makes life easier when doing this.

  11. #11
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    How hard is it to push the brass out of the 30-30 die? Doesn’t seem like fun to me.

  12. #12
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    No, I did not ream the case necks and that was an error on my part, so I apologize for forgetting to mention it. As for the boolit, I used an old Lyman mold that dropped 90+ grain boolits right at .319..........but I'll be darned if I can remember the mold number!

    As for getting the sized brass out of the 30-30 die, not a problem, it drives right out. Just remember to shove that LUBED case all the way in to the hilt.

    I was going to use 357 Sig brass also, but couldn't find any. However, our local PD just finished a practice session trying to hit the broadside of a barn (from the inside), so there were scads of .40 S&W laying around. It worked.

  13. #13
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    Quote Originally Posted by 3006guns View Post
    No, I did not ream the case necks and that was an error on my part, so I apologize for forgetting to mention it. As for the boolit, I used an old Lyman mold that dropped 90+ grain boolits right at .319..........but I'll be darned if I can remember the mold number!

    As for getting the sized brass out of the 30-30 die, not a problem, it drives right out. Just remember to shove that LUBED case all the way in to the hilt.

    I was going to use 357 Sig brass also, but couldn't find any. However, our local PD just finished a practice session trying to hit the broadside of a barn (from the inside), so there were scads of .40 S&W laying around. It worked.
    Ha!!! Just hosted the local police at my personal range. "Long range" carbine training (50 yards) was definitely entertainment! The pistol qualifier....well let's just say that I do believe running for it is a viable option.

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  14. #14
    Boolit Buddy

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    Quote Originally Posted by garandsrus View Post
    Do a search here, there are several posts on forming brass for 8 Nambu. I have formed it from both 357 Sig and 40 S&W. The 357 Sig would seem to be easier, but they both formed pretty much the same. One pass through the sizing die.

    I made a push through die to reduce the 40/357 Sig case diameter to Nambu size. It takes a stout press and of course case lube.
    Garandsrus: Can you share what diameter you made the push through die? The nambu rim is .413 and the head is .407. I’m assuming .413 and then you let the nambu sizing die do the rest?

  15. #15
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    I had to go a little smaller than .413 due to brass spring back. I think the die was .411 or .412.

    The Nambu die easily formed the shoulder with both calibers. I expected 357 Sig to work better, but it really didn’t.
    Last edited by garandsrus; 04-29-2021 at 08:04 PM.

  16. #16
    Boolit Buddy
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    here is a post I made some time ago, should help you out a lot. let me know if I can be of more help.

    Use an old 30/30 rifle sizing die with the the top of the die cut off, and get a lee bullet sizing ram that is close to the .357 sig case base size (I forgot what size I used) and use plenty of sizing die wax to size 357 sig cases by pushing them completely through the die with no need to hammer the cases out, as they pop out the top of the modified 30/30 sizing die with cases sized to the proper outside rim and body dimensions ready to form the neck by running through the 8mm nambu die. Then trim and chamfer deburr necks, anneal, and load as normal. Sometimes the original Nambu pistol extractor won't grab the rim of the thicker 357 sig case, and rather modifying your irreplaceable original go get another extractor from Don, (address below) you send him the top end of your Nambu and he will fit another extractor that is tailored to the .357 sig cases and not ruin your original for 40 bucks (he does not charge for the fitting). Veral Smith of LBT years ago made me a custom 4 cavity cast bullet mold that is the bee's knees for the 8mm nambu, I gun powdercoat them, and they shoot really nice, and makes cleaning the pistol a breeze. I'd give him a call. Let me know if I can be of any other help. https://www.nambuworld.com/donspartslist.htm http://lbtmolds.com/OurMolds/tabid/5807/Default.aspx
    Aloha No

  17. #17
    Boolit Buddy

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    Revisiting this thread. Thanks to a kind and generous forum member who made me a push through, I’m on my way to making Nambu.

    I think I need to take it a little slower with the nambu die stage. I started with crushed necks, and slowed down. Now getting slight splits. But progress!


    Click image for larger version. 

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  18. #18
    Boolit Grand Master
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    Seems to me that any major case reforming operation has a fairly steep learning curve, no matter how easy other people say it is.

    I usually figure on losing 10-15% of the first hundred cases just getting my dies adjusted and learning what works and what doesn't. If I lose less than that, I am happily surprised. More and I rethink my process.

    Except for the 6.5-06 from virgin W-W .30-06, I don't remember losing even one of those.

    Robert

  19. #19
    Boolit Buddy
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    Can a set of 8mm Nambu dies actually be found and purchased? Most places Ive looked are very expensive and out of stock at the same time. I saw a set from and action site that went for over $400. A friend has several and i would like to help him make ammo.

  20. #20
    Boolit Buddy

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    Quote Originally Posted by ebb View Post
    Can a set of 8mm Nambu dies actually be found and purchased? Most places Ive looked are very expensive and out of stock at the same time. I saw a set from and action site that went for over $400. A friend has several and i would like to help him make ammo.
    Ebb, I got mine by luck on flea bay. Pure luck. Maybe some day Redding/RCBS will do a new run for minor cartridges. Until then it’s thisi forum, gunbroker, and flea bay.

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