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

[Patrick Chadwick]

Regarding Norma brass, is it truely undersized or were the rifles specs not adhered to during manufacture??

As I have already mentioned, it seems from my measurements that Norma = CIP but CIP not= real-world Arisaka.

And as for some other types, most noticeably the Carl Gustav (Swedish Mauser), the rifles were made first, and CIP (or SAAMI) came later. I have a suspicion that the people who derived the specs took original values and then knocked off a not inconsiderable margin to be on the "safe" side. With the result that in the modern world, where every manufacturer lives in fear of liability claims, the manufacturers take the spec values, knock off another margin to cover worst-case manufacturing tolerances, and so we end up with noticeably undersized cases!

Patrick


---------- Post added at 11:01 PM ---------- Previous post was at 10:57 PM ----------

Twist rate:** 1-7.88''(early)

This wierd value is, of course, nothing other than 200 mm converted to inches (rounded up)

[jmoore]

Already found a rifle that seems to have good bullet fit with milurp ammo, but groove dia. roughly 0.265-0.266". Wierder and wierder...

[jmoore]

Next step? Chamber cast, anyone? I've cerrosafe but have lost it in the move, along w/ the digital camera. Errrr.

[Patrick Chadwick]

Jmoore, if you know anyone who is a muzzle-loader shooter, see if you can get a 36cal soft lead ball, then whack it down the barrel from the chamber end, using a 6mm brass rod to thump it. If the ball remains tight right to the muzzle, then you know there is no bell-mouthing, and the ball will take enough of an impression to let you measure groove and bore diameters, and see what kind of a profile the barrel has.

Patrick

[Parashooter]

A cast simply confirms that the chamber is indeed ~.010" larger at the base than issue ammunition. This seems not to have been a source of anxiety for the Japanese military, perhaps because the degree of case expansion is well within the elastic capabilities of good brass and there are advantages to providing clearance in combat comditions.

Properly-dimensioned sizing dies do not reduce the lower case body to as-new diameter, obviating concern about work-hardening in that region. Handloaders who object to the bulged appearance might consider using .35 Remington brass, which is inconsequentially shorter but "pleasingly plump", or obtaining a base-swaging die set to re-form .308 Winchester brass.


Left - .220 Swift, re-formed
Center - 6.5 Jap., Norma
Right - .35 Remington, re-formed

[jmoore]

Jmoore, if you know anyone who is a muzzle-loader shooter

Gee, that might be tough... Don't forget 00 and 0 buckshot for this trick- it's dead soft and nicely swaged, and the double naught works quite well out of my squirrel and coyote rifle...

[jmoore]

A cast simply confirms that the chamber is indeed ~.010" larger at the base than issue ammunition. This seems not to have been a source of anxiety for the Japanese military, perhaps because the degree of case expansion is well within the elastic capabilities of good brass and there are advantages to providing clearance in combat comditions. Properly-dimensioned sizing dies do not reduce the lower case body to as-new diameter, obviating concern about work-hardening in that region. Handloaders who object to the bulged appearance might consider using .35 Remington brass, which is inconsequentially shorter but "pleasingly plump", or obtaining a base-swaging die set to re-form .308 Winchester brass.

Nor was I concerned about base swell, personally, but what seems of greater import is the short shoulder drop dimension seen with some commercial cases. Norma particularly seems to back out primers, which seems to predict future case stretch at the web- IF the round is well seated forward at firing. No danger, but crappy case life, yes?

The 35 Rem example would be an argument against the semi rim being a reliable case locator/ headspace controller.

As you have some larger degree of experience here than us, what are your thoughts on proper bullet diameter? The Japanese military ran a considerably smller diameter bullet than what's being used today- and they could specify EXACTLY what they wanted for best results.

[Patrick Chadwick]

Gee, that might be tough...

Was that a "yes"?

[jmoore]

Shameless teaser post:

Had a T38 and a T97 at the range today (100yds- just getting warmed up). O-ring trick works, but isn't nearly as effective on the semi-rim as with rimmed cartridges. Groups a little better, maybe.

Milsurp ammo fired- only five- three good hits, more recoil and report than commercial, but no chronograph, two pierced primers (small but annoying), one hangfire with reduced report, and one dud (but it was an odd case mixed in on a loose charger full- the primer crimp way different!)

Photos and range report coming tomorrow, hopefully.- Too tired now! Up for 21hrs so far...

[jmoore]

Adding a link to the Range Report thread instigated by this one:

Japanese Ammuntion and Rifle Testing

For future reference it also may be found in Post #1 in this thread.