I believe that you're thinking of a Carcano.
There are Italian manufactured Carcano rifles in 6.5 Jap, called the Type I (I standing for Italian) these use a mauser type staggered row magazine and Japanese style dovetailed stock.
I believe these are rifled according to Japanese specs rather than using the Italian 6.5 bore (.268) and don't have progressive twist like the early carcano.
Later Carcano rifles used a non gain twist as well.
I've read that the Type I is the most accurate Japanese issued rifle, but few were used in combat, many were found still warehoused at the end of the war, probably due to lack of replacement parts or armorers trained in working on non Japanese designs.
National pride has been given as a reason, but I sort of doubt that.
Thinking on what I'd read about oversized Jap 6.5 bores best I can remember the examples cited were calvary carbines used in China. Not sure what model these may have been, possibly WW1 era rifles, perhaps something along the lines of the type 30.
In sources on Jap trainers they mention some fully functional rifles originally relegated to trainer use which were later issued for use in China when arms became in short supply.
There are some oddbal rifles built on a rare rejected action design that been shelved in favor of the 38 and 99 rifles, I think it was the Type 35 rifle, an improvement over the type 30 but with problems of its own which made the Type 38 a better choice. I think the rebuilt 35 was called a Type 45 and very few were made.
Many Jap rifles were rebored or rebarreled and otherwise altered by the Chinese and other once occupied nations. Chinese Arisakas can be found in 8mm and 7.62X39 and there are full military .30/06 conversions from some other country besides the conversions done by American owners. Mexico bought Arisakas in 7mm Mauser caliber.
It pays to be certain before trying Jap ammo since the rifles may no longer be in the original caliber.
