i've noticed this word "trimmed" in several mentions of this firearm.
In this case, "trim" refers to reloaders, using their tools to trim (shorten) a longer case to the desired length.
Pictures would certainly be a huge help.
Odds are that your Arisaka is one of two rifles, either the Type 38 (6.5mm) or the Type 99 (7.7mm) The Type 38 was the standard rifle until the Type 99 was introduced in 1939. Both types served throughout the war.
They use a modified Mauser design, the biggest difference is the firingpin and safety.
You won't find any "surplus" Japanese ammo. Unlike European rounds, Japanses military ammo was never brought to the US after WWII, in any quantity.
The 7,7x58mm (7.7mm Jap, 7.7mm Arisaka) is identical in performance to the .303 British. Same bullet, same speed. The cases are different, so the ammo is NOT interchangeable.
Commercial ammo is scarce and expensive. Handloaders can make 7.7 Arisaka brass from .30-06 cases by trimming the 06 brass to the right length, then running it through a 7.7mm sizer die. I've made quite a few that way. Sometimes you can find commercial 7.7 brass for reloading. I got some from Graf & Sons, but it was a long time ago.
Has your rifle been sporterized? Or does it have all its original features??
(again, pics would really help) Some features were dropped or modified during the production run, and can give a rough estimate of the time of manufacture.
The Type 99 had a number of unique features when introduced, some were dropped later in the war. The list includes a sliding dust cover over the action, the AA rear sight, and a folding monopod. Very rare to find the rifle with the dustcover, they were usually removed (and thrown away) by the troops, because they rattle. The monopod is kind of flimsy, and often was also discarded, to save a bit of weight. The AA rear sight has fold down arms, on each side, with notches in them, used for "leading" low flying planes. This was dropped from production during the war.
One thing to look at, is the receiver. Arisakas had the Imperial Crest (a chrysanthemum) on them. To "save face" this crest was ground off or defaced when the rifles were surrendered after the war. Not every one, but most. A rifle with an intact "Mum" is worth more to collectors, as this is a strong indication (but not absolute proof) that the rifle was a battlefield pick up, and not surrendered after the war.
One word about the "strength" of the Arisaka action. Ok, two words, they are, and they aren't. There were some made during the closing months of the war that are not quality arms, some are even reported to have been made of cast iron. These "last ditch" rifles are heard of more than seen, and should never be fired, if you do happen to have one.
The stories about the Arisaka action's strength comes from blow up tests done by P.O. Ackley, after the war. He discovered that Arisakas actually took more to blow them up than Mausers, or Springfields, or anything else.
HOWEVER, blow up strength is NOT anything useful its just the point where the action fails. Doesn't matter to the shooter if that point is 90,000psi or 112,000, both are waaay beyond what proper (service) ammo produces. Rifles can be damaged and rendered unserviceable by pressures well below the blow up point.
Enjoy the piece of history you have, and, see if you can find a bayonet to go with it. They aren't cheap like they once were, but a milsurp Arisaka just seems naked without one..,
