Need Help in Figuring out the caliber of a rifle.


October 12, 2008, 07:59 PM
Odd thing happened to me yesterday, and luckily did not involve any personal or material damage.

A number of years ago my, now deceased, uncle gave me a rifle he had been given by his uncle. The rifle is a Type 99 Arisaka, from the Nagoya Arsenal. It was missing a lot of the issue parts and I spent time getting the missing parts of the sights replaced, and a original dust cover, Nagoya bayonet, etc. Parts dealers who looked at the rifle said it was no big deal, nothing special. Just a 7.7 bring back with the chrysanthemum ground off. So not a battle field pick up, very little value other than the intrinsic value to me as an inheritance.

Problem came this weekend when I took it out to shoot if for the first time with a box of Norma 7.7 ammo. I checked to see if the bullet would fit into the muzzle end, to make sure it was not a 6mm rifle, and it looked dead on.

Loaded the internal mag with 5 rounds, chambered and shot the first round. Round went almost exactly where I had intended, and was happy with the light recoil and feel of the gun. First indication of a problem was when I had to repeatably beat the bolt back with my palm to get the round to extract. Another shooter looked for signs of overpressure and inspected the case, primer pocket fine, but the neck and shoulder of the round was blown out.

The chamber seems longer than that of the 7.7 Norma I was using. Took a round off, and it fell into the chamber and could not be picked up by the extractor. Had to use a cleaning rod to knock it back out.

Talking to my cousin later, he stated that he thought the rifle was a 7mm Mauser from boxes of old rounds left behind by his dad. Of course I never heard of this before last evening. The barrel is original, and not sleeved.

Questions, Was that a common conversion after the war? What is the best and cheapest way to determine what chamber I have? Why would it be re chambered for the 7 Mauser, when it might be closer in size to the 8mm Mauser which was common after the war?

Thanks for your thoughts.

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Jim Watson
October 12, 2008, 08:02 PM
It is possible to rechamber a 7.7 jap to .30-06. The chamber head diameter is oversize and the brass will bulge, but not split... often.
The heavy Arisaka rifling can spin up the undersize .308 bullet well enough for coarse purposes.

No chance of a 7mm Mauser working, nor an 8mm.

james rogers
October 12, 2008, 08:28 PM
I have a type 99 Nagoya that has also been rechambered. For decades Ammo was hard to get or expensive and the guns were plentiful. My dad had one to play with as a toy when he was a kid. They stuck firecrackers in the end of the barrel and lit them to simulate firing. They never saw or heard of cartridges for it. Other people of course figured out better ways to use them.

Mine retains the length of a 7.7 case but takes the contour of a 30-06.
They would cut down the 30-06 cases to the 7.7 length and load them with 31 caliber bullets. The 7.7 case is a slightly larger and the 30-06 would fire form to the chamber from there. When I fire a new 7.7 in it I can stand the blown out case next to a 30-06 case and it looks the same except shorter. Chucking them up in a mic shows the 7.7 to be only slightly larger in diameter.

There is quite a bit of information out there on the internet about using the 30-06 cases. I seen or heard of quite a few guns found to be rechambered this way when researching my own gun.

For me, I did not want to have to hand load that old gun, so I bought a beautiful barrel off of ebay. The chrome lined bore is like a mirror. Graf and sons has 7.7ammo and new cases if I want to reload.

If anyone wants a rechambered barrel let me know. I have a gunsmith lined up for this winter.

October 12, 2008, 08:30 PM
pics and measurements of the fired cases would help a great deal

October 12, 2008, 09:01 PM
The most common method is to make a cerrosafe casting of the chamber. You then can take measurements of the casting and make comparison of the #'s from a reloading manual or from online. has the cerrosafe. You just plug the barrel near the chamber and pour the low temp cerrosafe into the chamber and let it cool for a few minutes. It will drop out or it can be pushed out and checked.
Or you can have a knowledgeable gunsmith check it for you.


Dr. Tad Hussein Winslow
October 12, 2008, 10:01 PM
It's probably .30-'06 (7.5/7.7mm), not 7x57mm Mauser (7.0mm - wrong bore size). .30-'06 was a VERY common conversion of 7.7 Arisakas.

October 13, 2008, 01:55 AM
Thanks for the help. I will try to test fit an 30-06 round into the chamber tomorrow morning. If there is any problems, I may have to try the cerrosafe method.

From the recoil of the 7.7, I would have not thought the gun was built strong enough for the 30-06 round.

Krochus, unfortunately I don't know where the spent cases are. I looked for them this afternoon and am wondering if they are still at the shooting site. A number of folks were handling them, and I think someone did not put them back in the bag. But if I find them, I will post.

October 13, 2008, 12:10 PM
Hah, the 7.7 Arisaka not strong enough for a .30-06?

The Arisaka is arguably THE strongest and best-vented military bolt-action produced.

October 13, 2008, 02:41 PM
Vaarok, I am not an expert, but it just seemed that my 03A3 is a lot better built rifle, and a more powerful round.

Good news, I think, is that I test fitted an 06 round into the chamber and it fit snugly. Only took a little pressure to close the bolt.

What is my next step, I don't want to fire it until I know that it is safe. I am kinda leaning toward a gunsmith inspection.


Ron James
October 13, 2008, 02:58 PM
:)Ahhhh Acera, Vaarok didn't say it shot a more effective round, nor was pettier, nor was easier to handle nor built better, nor had better balance, in fact I think it's down right ugly ( of course beauty is in the eye of the beholder ), he said it had a very strong action, which it has. It seems the Japanese did at least one thing right in their rifle design. :)

October 13, 2008, 03:42 PM
I'd tie it to an old car tire and fire a few 30-06 rounds with a 50' lanyard cord.
(Put the butt in the tire and tie the forend down to the other side of the tire.)

You then will have some fired 7.7/06 cases to look at with no risk to yourself.

If everything looks O.K. and it doesn't blow up, it is up to you then whether or not you want to fire it from the shoulder.

Myself, I'd take it to a gunsmith along with the fired cases, and have him check the headspace with a 30-06 headspace guage just to be safe.

Or safer!


james rogers
October 13, 2008, 07:09 PM
What you might have is a 31/06, the bore was 31 caliber on a 7.7 like on a british 303. My guess is that 30 caliber bullets are not going to snug the grooves very well. A 31/06 might be pretty cool though.

Dr. Tad Hussein Winslow
October 13, 2008, 07:21 PM
I am kinda leaning toward a gunsmith inspection.

Yes, absolutely!

October 13, 2008, 11:38 PM
The best story I've heard about the strength of the Arisaka action involves a 6.5mm rifle rechambered to 30-06 but retaining the 6.5mm bore. :what:

First 2 rounds were hard to extract, 3rd locked it up tight. Gunsmith figured out the mistake. IIRC, the 6.5 Jap is approx .257, the .308 bullets were being swaged down on firing. If that action withstood THAT kind of overpressure, a 7.7 can handle 30-06 without breaking a sweat.

But take it to a good gunsmith just to be sure of exactly what you're dealing with.

October 14, 2008, 06:26 AM
I am kinda leaning toward a gunsmith inspection. Lean heavily and then go to the smith.

Gun smiths are just so much cheaper then surgeons and hospitals are.

Dr. Tad Hussein Winslow
October 14, 2008, 08:45 AM
The best story I've heard about the strength of the Arisaka action involves a 6.5mm rifle rechambered to 30-06 but retaining the 6.5mm bore.

Yep, my understand is that rifle is in the NRA Museum in Fairfax. Supposedly ol' boy thought it was 7.7 bore, not 6.5, so he reams to .30-'06. Loud boom, poor accuracy, hard extraction - what's going on? Shoot it again, same thing. The bullet was swaging down to 6.5mm, and the gun was fine. I sure would like to know WHICH .30-06 ammo he was using. It'd be double impressive if it was a 220 grainer.

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