Both versions of 20mm Type 99 were available with a 131g API containing 3.4g incendiary material in the base. The Type 99-1 fired these at around 600 m/sec, the Type 99-2 at 750 m/sec.
At short range, and striking at the most favourable angle, the 99-1 could penetrate around 20mm, the 99-2 probably 25+mm, although in the normal conditions of ground attack (i.e. at longer ranges, and striking the armour at a glancing angle) those figures would be reduced significantly.
Most AFVs in the Far East were smaller/older/less well protected than those in the European sector. On test, the 20mm Hispano SAPI (130g at 880 m/s) could get through Japanese armour without too much trouble. So the Type 99-2 at least could be a danger to many of the tanks in theatre.
Tony Williams
Author: Rapid Fire - The Development of Automatic Cannon, Heavy Machine Guns and their Ammunition for Armies, Navies and Air Forces.
Details on my military gun and ammunition website:
http://www.users.globalnet.co.uk/~autogun/