Modern firearms have two modes of firing: single shot and rapid fire. Single shot attacks use standard combat rules: roll a d20, adding the attackers Base Hit Bonus and Dex modifier (if applicable) versus the target’s AC. As you would guess, a single shot attack only uses a single round of ammunition.
Rapid Fire attacks fire several shots at once (minimum 2 shots), up to 10, 20 or 30 rounds, determined by the type of weapon used. RF attacks are not made against AC, but instead involve rolling a d10 for each shot fired: each d10 that rolls a 1 scores a hit on the target regardless of its AC. Roll damage for each hit scored. Each shot fired consumes a round of ammunition. Attackers may divide the number of shots fired between different targets. Simply divide up the dice and roll each batch separately.
- Combat Pistol – 1d6 damage, range 30 feet, clip 15 rounds, wgt: 3 lbs., Rapid Fire 10
- Submachinegun – 1d6 damage, range 40 feet, clip 30 rounds, wgt: 6 lbs., Rapid Fire 20
- Combat Rifle – 1d6+1 damage, range 70 feet, clip 30 rounds, wgt: 8 lbs., Rapid Fire 20
- Machinegun – 1d6+1 damage, range 100 feet, belt 100 rounds, wgt: 15 lbs., Rapid Fire 30
Example: A SMG can Rapid Fire 20 rounds in a single combat round (note: this is actually not very accurate for a 1 minute combat round, but some accommodation must be made for D&D’s abstract combat mechanics). Assuming a player decides to go all out, they consume 20 bullets and roll 20 10-sided dice. Each dice rolling a 1 scores a hit on the target(s).
Creatures only hurt by silver weapons still require silver bullets to harm. For those creatures only harmed by magic weapons, it’s left to the GM’s discretion whether modern firearms are equivalent to magic weapons, or whether actual magic bullets are required.
Firearms are quite loud. Nearby monsters should be immediately alerted when guns are fired. GM’s may also want to consider making an additional wandering monster check every time firearms are discharged. GM’s should also consider whether sound suppressors are available, and what the trade offs for using them should be.
One final consideration, ammunition should probably be rare or fairly expensive. Otherwise, your players will just load themselves down with ammo and blaze away at everything, relying on “spray-n-pray” to win every battle.
Obviously, these rules should be applied sparingly against the PCs, especially at low levels. Conversely, even low-level mooks armed with assault rifles can make effective adversaries against high level PCs.
Optional Rule: Mercenaries lose the Combat Machine ability and replace it with the Burst Fire ability, allowing them to roll d8’s to score hits, instead of d10’s.
Optional Rule: Modern firearms can be reloaded very quickly, especially so in the context of a 1 minute combat round specified in Swords & Wizardry Whitebox and White Star. So, you may wish to “hand wave” reloading of firearms, assuming the PCs are proficient enough to reload very quickly and keep blazing away. This will have the effect of reducing some of the bookkeeping involved, essentially ignoring a gun’s ammo capacity.
Optional Rule: You may want to adjust the target number to score a hit based on range. For example, rolling a 1 to 3 if the target is adjacent, 1 or 2 at short range, a 1 at long range, and 0 at extreme range.