Okay, here's what the rules say about this in section 12.3:
1. The Japanese fails to cause any CP/AP during an Assault, a Defensive MG Fire, or a Ranged Fire. (See 12.5)
2. They roll less than or equal to the Ammo Number that is in the active Ammo Box during any Assault, Defen- sive MG Fire, or a Ranged Fire attempt. If on Ammo Box 3, the DR must be less than the Ammo Number.
3. They Pass and subsequently roll less than or equal to the Ammo Number in the active Ammo Box.
Note that the word 'modified' doesn't appear anywhere, so I've assumed it's the natural roll.