WI: US has not adopted a semi-auto rifle by its entry to WWII

For one thing, the barrel length for the numbers on Wikipedia is longer for the Arisaka than the Carcano, so that matters somewhat. Also the numbers on the page for the Type 38 Arisaka rifle are lower than on the ammo page even with the same barrel length. I think it is likely that they were roughly comparable in performance from similar length weapons and both seem to rely on very long barrels to get their performance.

There were short rifle and long rifle variants of both the Type 38 (6.5x50mm) and Type 99 (7.7x58mm). One of the reasons the long rifle persisted longer was that there was considerable emphasis on bayonet training in the IJA, and one of the requirements for the Type 38 was that it had to allow a Japanese infantryman who was expected to be shorter than a Westerner pierce the abdomen of a cavalryman when the Type 30 Bayonet was fixed.

Snipers (IIRC) also particularly valued the long Type 38 and reduced-charge machine gun ammunition because the combination of light powder load and long barrel ensured greatly reduced flash and report and made it much easier for them to remain concealed.
 

Deleted member 1487

There were short rifle and long rifle variants of both the Type 38 (6.5x50mm) and Type 99 (7.7x58mm). One of the reasons the long rifle persisted longer was that there was considerable emphasis on bayonet training in the IJA, and one of the requirements for the Type 38 was that it had to allow a Japanese infantryman who was expected to be shorter than a Westerner pierce the abdomen of a cavalryman when the Type 30 Bayonet was fixed.

Snipers (IIRC) also particularly valued the long Type 38 and reduced-charge machine gun ammunition because the combination of light powder load and long barrel ensured greatly reduced flash and report and made it much easier for them to remain concealed.
That would certainly help explain the 31 inch barrel.
 
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