7.7x58 will not chamber?

i got a arisaka 99 that has tha mum stamp,nagoya,,,but 7.7x58 will not chamber,,,a 303 will go in about 2/3 tha way,,,,it has only 1 gas relief hole below tha mum,,,tyvm,,,

Unless there is an obstruction in the chamber, perhaps it has been converted to another caliber.
Chinese conversion to 8x57 or 7.62x39? Korean to .30-06? A chamber cast might be appropriate.

Did you load the round in the magazine and then close the bolt? The Type 99 has a mauser type extractor. This means you can not just drop a round in the chamber because the extractor cannot snap around the cartridge rim. The cartridges need to be loaded from the magazine.

If it does not work by loading from the magazine, suspect that the barrel has been converted to a new caliber.

I apologize if this response seems too basic, but I don't know your level of expertise.

Walter

More than a few converted to .300 Savage out there.

Al

Welcome to the forum, yes a chamber cast seems to be the way to go here...

Welcome to Milsurps, as has been said by Jim and others the definitive answer will be gained by a chamber cast and slugging the bore.

Although it might not help either, we'd love to see pics of this one too...it might help... Mostly we just want to see.

If it was rechambered to 30/06 , or 8mm a 7.7 will still chamber . If it was redone to 300 Savage the barrel shank will be shorter . If it has a non Japanese barrel on it you should be able to tell by looking at it .

Could also be a separated case in it, anything could be wrong. It needs a look see...

Mostly Type 38s that I've seen, but wondering if it wasn't converted to 7,62x39? The fact that .303 SAA fits relatively well but is short supports that guess.

How would one go about converting a T-99 to 7.62x39 ?

The Chinese did the Type 38 conversions and they swapped the barrels out.

The type 38 is different than a type 99 . All the Chinese type 38 7.62x39 conversions I have do not have a different barrel , just the original bored out and set back and rechambered . The type 38's bolt face will work with the 7.62x39 base size .