The Japanese used Captured Chinese (FN made) BARs both in China and possibly in the Phillipines. These were in 7,9mm, NOT 7,7mm; The Type 99 7,7 Rimless round was probably not in General field issue in the Pearl Harbour period of the war...All photos of Japanese Invasion troops in Malaya-Singapore, the Phillipines, and New Guinea in early 1942 show the 6,5 T38 rifle predominating.
It is known that the IJN also used Chinese Type 26 (ZB) machine rifles in the Phillipines campaign.
Quantities of 7,7 calibre rifles and LMGs only became evident in the Pacific TO in late 1942. The 7,9 round used by the Japanese is easily mistaken for the 7,7 at simple inspection ( NO headstamp, but staked primer crimp, not circular). I have found samples of Japanese 7,9 ammo mixed in with 7,7 T99, and sold as such ( from New Guiniea Pickups, usually at airfields, where 7,9 was used in aircraft MGs ( ?Type 98?).