I can help here Shinpachi, even though I cant read Japanese (which I'm ashamed, because I really want to
) as luckily I have some manuals translated by the ATIS.
In regard to Ki-61 the maintenance manual titled "Ki-61 Handling Reference" published by Army Flight Test Department in 1942 for the prototype. I have indicates loads for a configuration with 2 x 12.7 mm machine cannons and 2 x 7.7 mm MG's (standard for Ki-61-I Ko) :
"Chapter III - Equipment
Section 1 - Armament
Number of rounds carried
Position Gun Type Rounds carried
Fixed fuselage machine cannons - Ho-103 fixed machine cannon - 600 rds
Fixed wing MG's - Type 89 Fixed MG - 1000 rounds
(Note: 1000 rounds may be carried for the machine cannons.)
I'd assume that maximum capacity of Ho-103 would be 500 rounds per gun in overload, especially as gun section indicates further :
"All necessary equipment such as magazine, link and cartridge chutes are installed. 1000 rounds are easily carried."
"The magazine are easily removed from or attached to the side of the fuselage. They hold 500 rounds each, and are constructed so that the ammunition can be pulled out of the top."
But standard load would be 300 rounds per gun (I guess).
What was the load of the wing mounted 12.7s I dont know, it is possible that its 250 rounds as most books and materials indicate.