5.5×40mm Arisaka-Chiotanne

The 5.5×40mm Arisaka-Chiotanne, also known as 5.5×40mm Western, is a military rimless bottlenecked intermediate cartridge developed in the late 1970s in Yamatai and Chisei by Arisaka and Chiotanne. It is the the second rifle cartridge standardised for HECO forces under the HECO Joint-Arms Cooperation Treaty, after the 7.7×58mm Arisaka, and is also widely used around the world.

A "heavy" variant exists, designed specifically for squad automatic weapons and designated marksman rifles, with the same dimensions as the standard 5.5×40mm, but utilizes a longer streamlined bullet with a heavy steel core for increased performance at extended ranges and penetration.

History
In 1967, the 7.7×58mm Arisaka rifle cartridge was selected as the first standard rifle cartridge of the Three Powers Alliance, in preparation for the planned invasion of Hinomoto. At the time of selection there had been criticism that the recoil power of the 7.7×58mm Arisaka, when fired from a hand-held lightweight modern service rifle, did not allow a sufficient automatic rate of fire for modern combat. The adoption of the 7.7×58mm Arisaka was also due to unrelated political reasons. As the entire Yamatai Imperial Army was already armed with the 7.7×58mm Type 20 rifle, and Yamatai threatened to withdraw from the standardisation project should a different round be chosen. The Yamataian government also produced a large amount of questionable studies proving the superiority of the 7.7×58mm Arisaka to is competitors.

Yeongseon had extensive evidence with their own experimentation with intermediate cartridges since 1945 and were on the point of adopting a 7 mm cartridge when the selection of the 7.7×58mm Arisaka was made. The concerns about recoil and effectiveness were effectively overruled by Yamatai leveraging its strategic position in the planned invasion of Hinomoto, and the other Alliance nations accepted that preserving the shaky regional alliance and equipment standardization was more important at the time than selection of the ideal cartridge.

Combat experience in the Hinoan War soon proved that the 7.7×58mm Arisaka was indeed too powerful for accurate automatic rifle fire, resulting in a below-average hit probability in combat situations. Upon the conclusion of the Hinoan War, the newly-formed HECO decided to pursue development of a new, standardised intermediate cartridge size to replace the 7.7×58mm Arisaka. Of the cartridges tendered, a prototypical 5.5×40mm round designed by the Chiotanne Group was the basis for a new design created by Arisaka. The new cartridge was selected for use by HECO forces, produced as the 5.5×40mm Arisaka-Chiotanne and with an overseas production designation of 5.5×40mm Western.

While Chisei and Yeongseon quickly adopted the round with the, political resistance in the Yamataian military and government continued well into the 1980s. It would only be until the adoption of the Arisaka Type 45 rifle in 1989 would Yamatai widely adopt the 5.5×40mm Arisaka round for its standard rifles, though the Morita Type 38 light machine gun was chambered in 5.5×40mm Arisaka and began wide use in 1985.

Yamatai
In Yamatai, the 5.5×40mm Arisaka-Chiotanne is manufactured by Arisaka and its subsidiaries. The Yamatai Imperial Army uses the 5.5×40mm Arisaka-Chiotanne in its main assault rifle, the Arisaka Type 45, as well as the Morita Type 38 light machine gun.

Worldwide
The 5.5×40mm Arisaka-Chiotanne is widely used around the world, and is also produced under licence by numerous foreign corporations under the name 5.5×40mm Western. A large amount of unlicensed production and copies of the round also exist.