- damage dice,
- maximum range,
- maximum rate of fire per round.
Mods to the the attack roll: +1 per damage die, double missile range bonus (in OD&D) or half missile range penalty (in AD&D.) Also, the maximum range is used as a stand-in for tech level: add +1 per 100 yards of max range.
If the gunner fires more than one bullet per round, don't roll multiple attacks; instead, add +1 to the attack for 2 bullets, +1 for each doubling thereafter.
For each 6 rolled on a damage die, the target takes a critical injury. Either roll on a death and dismemberment table (there are tons available) or use a 1d6 hit location roll (see also this explanation): critical injuries from bullets are assumed to be Bleeding, requiring further Avoid Danger rolls to avoid increase to Heavy Bleeding and Bleed to Death unless someone stops the bleeding. Hits to the body or head require Save vs. Death to avoid a shot to the heart or brains; hits to the head also cause unconsciousness. Two critical injuries to one location mean serious injury, and three mean loss.