This is a simple house rule to give firearms a bit more differentiation. These rules assume that “firearms” in White Star represent modern or futuristic ballistic weapons.
Firearms may be fired in one of two modes: single fire or rapid fire. A single fire attack makes a standard d20 attack roll against a single target. A rapid fire attack is more of a “spray-n-pray” approach: if you fire enough bullets something should stick.
For each round fired in rapid fire, roll a d10: if the result is a 1, that round hit the intended target, regardless of the target’s actual AC. Firearms can fire a maximum of 30 rounds in a rapid fire attack, and these may be spread between adjacent targets as desired; roll separately for each target.
As a further optional rule, you may want to rule that Mercenaries lose the Combat Machine class ability, and instead gain the Mow ‘Em Down class ability, which allows Mercenaries to roll d6’s for rapid fire, instead of d10’s. This would make Mercenaries more ranged combat oriented, while typical Fighters would be more melee oriented.
However, there are important implications for adopting such rules. For one, you probably don’t want to use rapid fire attacks against a low-level party; they’d be wiped out pretty quick. The flip side of that, of course, is that with firearms, even low-level enemies can be deadly threats to high-level PCs.
Also, depending on the availability of ammunition, firearms could quickly out-class laser weapons, so that is another consideration. And remember that ammo is bulky; the party should only be able to carry so many bullets!