It should've been on p.112 or 160, but it seems to be missing.
Basic autofire: every 2 pts you hit by, you add +1 to damage*, up to a maximum of +5 (you would have to beat the target's Defense by 10) or half the attack's normal bonus rounded up (e.g., a +5 autofire could be boosted to +8, if you hit by 6); this is a +1 extra.
You can take another +1 extra to reduce the interval to 1 (so, for every point your attack roll beats the target's Defense, you add +1 damage -- same maximum damage bonus, though).
Another +1 extra increases the max to +10, or the attack's normal bonus, whichever is less.
You can kind of figure it out from the 2nd & 3rd paragraphs ("reduces Autofire interval (the amount your attack roll needs to exceed the target's Defense) to 1 instead of 2", and "maximum bonus from autofire is +5 (for exceeding the target's Defense by 10 normally").
*Note that bonus damage from autofire doesn't help in bypassing Impervious. If your target has Impervious 10, and you're shooting him with an an machine blaster with base damage +8, even if you have Autofire 3 & hit by 8, doing +16 damage, it bounces -- the base damage is less than his Impervious.
