FIREARMS WERE NOT ABLE TO PENETRATE FULL PLATE. AT ALL. Maybe at close range, if you were lucky.
Well, I’m not an expert, but according to wikipedia they actually were able to penetrate plate armor:
They [the handgonnes] had superior armor-penetration capability; the longbow was effective against mail armor and plate, thanks to the bodkin point, and the crossbow very effective against heavy armor, but the hand cannon could pierce heavy plate as well
The German wikipedia article says, the projectile could penetrate a knight’s armor within 100m.
That would’t be close range any more. This weapon wasn’t effective at far range only because of its bad accuracy.