Ranged combat in Moldvay/Cook still gives me fits. Yeah, it works well enough, but only being able to shoot once every ten seconds is insane, even for crossbows. Things get worse when you start looking at adding modern or futuristic firearms.

When translating firearms to D&D, most folks start with the damage. They want guns to be as dangerous as they are in real life, so turn to tools like exploding damage dice. This certainly does make firearms dangerous to high-level characters. However, that doesn't quite work with my sense of hit points being the ability and will to continue to fight, rather than structural integrity. Traditionally, fire arms were seen as the great equalizer because they rendered strength, size, expensive equipment, and, in many respects, skill, largely irrelevant. Anyone with a modicum of training and steady hands could too easily slay a knight in the finest armor, atop the most expensive horse, and with a lifetime of training. "The good Lord made all men," as the saying goes, "but it was Samuel Colt who made us equal."
So I'm thinking of approaching fire arms from the other end, and giving them bonuses in the attack roll. These bonuses are going to be pretty hefty. Even a simple firearm like a matchlock or flintlock deserves a +1 or +2. Once you're talking about shotguns and full-auto machineguns, bonuses of +5 or more may not be out of the question. (Keep in mind that the spread of a shotgun and the spray-and-pray firing technique of an assault rifle on full auto are less about putting lots of holes in your target and more about putting lots of lead in the air in order to increase the chances of hitting something.) Firearms will likely do about the same amount of damage as other weapons, since losing hit points is more about stress, distraction, and exhaustion than it is about physical integrity and blood loss.
Because a certain someone is obsessed with gauss weapons, we'll be taking a look at those here. The really cool thing about these weapons is that they allow a wide range of ammunition types. So let's see what we can do with that:
Solid Slug - the most basic projectile. This grants a +3 on the attack roll, +5 with autofire (but keep in mind that this increases the chance of hitting something else as well). It has an effective range of 1,200 feet and does 2d4 damage.
Smart Slug - this is a solid slug with wings. It can’t shoot around corners, but it is far more accurate. It can only be fired in single-shot mode, enjoys a +4 bonus on the attack roll, and an effective range of 2,400 feet. However, this projectile requires additional gear for properly selecting a target. It also does 2d4 damage.
Explosive Slug - a solid slug that goes boom. Using the same targeting gear as the smart slug, this one actually can reach round corners. When it goes off, roll a normal attack roll for every target within a 5 foot radius, adding +3 to all the attack rolls. Those hit take 1d6damage. Autofire is not an option.
Screamer Slug - a solid slug that whistles and then goes boom. The screamer slug has an effective range of only 900 feet. It doesn't require any special targeting equipment. However, it emits a hideous whistle as it flies through the air. It also explodes after it hits the target, doing 2d6 damage after a successful hit. The combined effect is a -2 on the morale rolls of anyone being shot at. However, the wonky aerodynamics give it only a +2 on the attack rolls, +3 with autofire.
Force Slug - a slug that pierces force fields. This is very expensive round generates its own force field designed to penetrate defensive force fields. It has an effective range of 900 feet, but is otherwise just like a solid slug.
Taser Slug - a slug that shocks. This acts just like a solid slug, but only does 1d6 damage and doesn't kill your target.
Needle Shell - a disintegrating shell that releases a swarm of tiny needles. The effective range on this is only 200 feet. The attack roll bonus is +4, +6 on auto fire. The needles can be used to inject a wide variety of toxins into the target. Usually, this at least doubles their cost.
Painter Shell - this shell splatters a gooey substance on the target that calls out to smart slugs. It adds an additional +2 attack bonus to Smart slugs fired at the same target. It only does one point of damage, but is otherwise just like a solid slug.
Am I missing any good ones?
Art by Ludovico Marchetti.