6,5mm Japanese Arisaka Ammuntion Dimensional Revelations (Photo Heavy)

[jmoore]

16 May 2011 ETA: see also the thread link below as it will tie into this one:

Japanese Ammuntion and Rifle Testing


As part of an investigation instigated by a thread on the Japanese Rifles forum, I decided to actually check some dimensions of some milsurp and some commercial ammuntion.

Three highlights and then mostly pictures.

First: Original bullets (four different samle types) AREN'T 0.264" diameter. More like 0.2615-0.262".

Second: Some commercial ammo is seriously undersized on the rim thickness and drop to a shoulder datum. (pick whatever datum you want- its still short!)

Third: Base dimensions for all ammo checked are fairly uniform but small for the chamber. (This is what started this knowledge quest originally!)

Blue million pics!:




1a, 1b. Milsurp ammo


1c.


1d.


1e.


1f.


1g.


1h.

That'll do to start. More coming...

Information

Warning: This is a relatively older thread This discussion is older than 360 days. Some information contained in it may no longer be current.

[Patrick Chadwick]

That'll do to start. More coming...

Yes please - getting my Arisaka to perform well is at the top of my "to-do" list, and the Norma ammo I have is obviously seriously undersized for the chamber (which is perfectly OK - no rust, scratches or whatever).

Patrick

[jmoore]

Rim thicknesses are also not consistant. Note that the Norma ammo is not the newest, but I've lots of it and have had no previous reason to buy more.

Thicknesses were measured with a vernier height gage on a shop grade granite surface plate. An average of three drops were taken on each case measured and checked against several other samples of the same type of ammo, if available. Note that the bases on the milsurp ammo pictured in photo 1b. are somewhat rounded and have strong primer stakes. If particularly proud, the stakes were gently filed flush to prevent false readings. The dome like nature of the bases prompted the use of the height gage rather than a normal quick "mike-ing" of the rim thickness.

Not a very photogenic process, no photo at this time.

Results:

Milsurp: 0.052"
Hornady/Grafs: 0.052"
Norma: 0.043"

Base diameters:


3a.


3b. AKA Hornady


3c.

Overall case lengths aren't hugely critical unless they are too long, but it does affect the "name" of the cartridge if done as follows- 6.5x50mm Japanese- Kamzac did note that 6,5x51 seems a better name, but it's not commonly used even if it's much more correct!


3d.


3e. AKA Hornady


3f.

Needed to measure a drop to the shoulder, as I don't altogether trust the teeny rim to do it's headspacing function, given large chambers and any possible chamber mouth radius!

As a purely comparative measure, not intended to meet any "standard dimension", I used a Redding neck sizing bushing as shown below:


3g.

Results- Surprising!:


3h.


3i. AKA Hornady


3j.



Member kamzac has other info, and can be credited with instigating this thread, along with ireload2.

[vintage hunter]

Some more measurements for you JM. The ammo being measured is definitely WWII Japanese. 1.Bullet dia.:0.261'' 2. Base dia. :0.4475'' 3. Rim dia. : 0.4735'' 4. Rim thickness: 0.049'' 5. Case length: 1.999''. Had some milsurp ammo once that was said to be Chinese manufactured during the Korean War, don't have any cases to measure but it had a 1950 headstamp and bullet dia. was 0.257''. Awful stuff.

[jmoore]

Thanks vintage hunter!

In reference to photos 3g through 3j, I did the same test on a fired Norma case (A rare one without a backed out primer.) Drop dimension measures 2.030" -only about 0.008" more than the unfired milsurp, but near on 0.020" over the commercial cases!

Some more dimensionals;

Cartridge overall length:

Milsurp ammo type sorted by visual cues:
Brass bullet- ________________2.972"
Gilding metal-_______________ 2.999"
Cupro nickel w/ staked primer-_ 2.993"
Cupro nickel no staking-______ 2.995"

Commercially manufactured ammo:
Norma FMJBT-______________ 2.804"
Hornady PSP-_______________2.770"

Oddly, the bullet length of the Hornady and the milsurp ammo are pretty similar, but the cannelure placement is very different:


5a. Hornady (top) and "brass" milsurp bullet


5b. Norma

The boat tail design of the Norma bullet would preclude it from being seated out far enough to match the original, but why the Hornady specialty ammo is made so short is kind of a mystery. No great task to do a run of bullets with the cannelure in the "proper" location considering all the other efforts taken...

Rim diameters:

Milsurps run between 0.474-0.477"
Norma-____________ 0.467"
Hornady-__________ 0.471"

[jmoore]

I'm curious if anyone has measured the type 38 bore accurately enough to precisely calculate the cross sectional AREA of the bore. Complicated geometry to be sure, but I wonder if that might not be the best way to properly match a particular bullet diameter, since the "lands" don't inscribe the bullet like more conventional rifling.

Running these 0.264" bullets down what may be a bore designed for smaller bullets may be why it's hard to get super accuracy out of these rifles.

I may EVENTUALLY pull some milsurp bullets and reload them into new cases, but mostly they'll be those odd brass colored bullets. Reconstituted ammo just never seems to be all that great, though.

[vintage hunter]

A friend has an old book that has the actual Japanese specs for the 6.5mm Arisaka rifles and ammunition. Going on memory here so some figures could be wrong, the bore diameter was to be .257-.259. Groove dia. .262- .265. Bullet dia. .261-.263. It also had information on rifling twist, powder charge weight, muzzle velocity and several other tidbits. I believe the book stated that in 1925, or there bouts, Japan switched from the 155ish grain RN bullets to the 139gr spitzer and the number of grooves was changed from 6 to 4 and the the twist rate was slightly increased. It may have also included the bore area figures too. Been a long time since I read it and as soon as I remember which friend has the book I'll read it again and update the info. More later......

[jmoore]

Eagerly awaiting the additional info, vintage hunter!

Due to the short shoulders (and thin rims) on the new cases, it seems that the o-ring trick to keep .303"SAA cases tight up against the bolt face might work here as well when utilizing new brass. Backed out primers as are common when shooting Norma cases means excessive brass stretch sooner or later, no?

[Patrick Chadwick]

My CIP-derived information gives bore 6.33 mm = 0.249" and "groove" diameter 6.63mm = 0.261. Twist is given as 200 mm = 8"

But since the Norma cases I have appear to be CIP-compliant, but are clearly too small for the chamber, I would not trust these specs overmuch!

Patrick

[vintage hunter]

Ok, have the book. Specs for type 30 and type 38 rifles and carbines: Bore diameter-.256''. Groove diameter-.262''-.269''** Twist rate:** 1-7.88''(early) to 1-9''(late). Number of grooves: 6(early) or 4(late) Cartridge specs, 6.5mm, type 38: O.A.L.-3.00'' bullet dia.:.262'' Weight: 139gr, Length: 1.28'' Powder type: Nitrocellulose, Charge: Approx. 33gr, MV: 2400 fps, Pressure: 38,000 psi, Case lenght: 2.00'' oal, Length of shoulder: 1.53'', Dia. at rim:.4761'', Dia. at neck: .291'' Length head to shoulder: .20'', Length of neck: .27'', Diameter at base: .449'', Diameter at shoulder: .414''. ** Notes...Groove depths and rates of twist have varied considerably with time and place of manufacture** JMHO but the variation in groove depth/diameter without an increase in bullet diameter is the likely cause of some rifles lacking in the accuracy department. Regarding Norma brass, is it truely undersized or were the rifles specs not adhered to during manufacture?? Do they stretch? Don't think so, never had a head seperation with Norma but I have with Graf/Hornady brass. (not PRVI/HORNADY)