Press [F10] on the high-score table and the words 'High scores' at
 the top will change to 'Firm help'. You can then use [F7] during the
 game to improve health and get extra ammo. [F8] to skip levels and
 [F9] for invincibility. 
 
 Extra ammunition:
 Shoot the animals (eagles, pigs, etc.) during game play to 
 collect more ammunition.