ACAMS Development Blog

In ACAMS, we want to have a balance between stark realism and gameplay, which is why we had to make a few sacrifices in the medical department. Probably a few pigs and maybe a lamb or two. There are seven total body zones: head, upper torso, lower torso, each arm, and each leg. 

When you shoot someone, depending on where you shoot them, you have a chance of hitting an organ if you penetrate their armour. There are some compromises, however, since our concept of a hitbox is so limited: you can’t hit the arms and torso individually. Instead, because you’re usually holding a gun up or at the chest, you have a chance of hitting the arms instead of chest from the front.

When you hit someone, the system rolls some dice. Depending on how close to the center of mass you hit them on that bodypart, it has a higher or lower chance of doing closer to full damage for that round type. Each body part has some percentage of “health”, which is the relative usability of that part, with the exceptions of head and chest: if you lose either of those, it’s game over. If you lose all of your other limbs, you’d be a motionless, useless torso until you die of blood loss, but you’d still be alive, if probably unconscious from pain.

Yup, that’s right: there’s going to be pain. When you get shot, it hurts. A lot. You yell in pain, your vision goes blurry, and kind of dark around the corners, but you’re still alive, usually. Where you get shot affects your performance. If you get shot in the arms, you can’t aim as well, or not at all if they’re totally disabled. In the legs? Well, sorry buddy, but you’re going to have movement problems. 

When you get shot in an organ, a few things happen. For your lungs, it hurts a lot and you can’t breathe as well, meaning that your stamina is basically nothing at that point. Liver? Same deal, but now you’re bleeding a ton. Heart? Dead. Brain? 50/50, you may die, or you may just not be able to see much.

Luckily, armor exists.

The thickness of the line around your body indicates the level of armor you have on it. It’s assumed by default that you have at least Level II-equivalent armor, else why are you walking into a warzone? This goes all the way up to Level IV armor. This of course affects your weight, which in turn affects your movement speed and stamina, but could make you less dead. It’s a tradeoff.

The color is your body part state: green is okay, yellow is injured, orange is severely injured, and red is critically injured. Grey (not shown) is disabled, meaning that it’s severed or otherwise rendered useless.

Materials in ACAMS will be represented by a fairly mathematically sound way of thinking about it in a way that it would be actually affected mechanically: ultimate tensile strength.

Why ultimate tensile strength instead of compressive strength? That’s a good question. When an object impacts another object, assuming (for the purposes of demonstration) that the impacting object is perfectly rigid and can’t be deformed (as we represent bullets for reasons of computational load), the object we are impacting with is deformed, bowing outwards from the point of impact, creating tensile stress. This is readily apparent in the case of aramid fibers. When you shoot a layer of aramid fiber with a bullet, it bends and then the fibers snap from exceeding their ultimate tensile strength.

The way we approach this isn’t the most physically accurate method, but it is Close Enough™ for representing real-world material properties and interactions.

(where t = .0001)

We represent our deceleration time upon impact as .0001 seconds because that’s what seems to give the most accurate results without again becoming computationally expensive. I hate to make these kinds of tradeoffs, but you’re pretty limited with 64KB and a vanishingly small fraction of a CPU thread to play with.

These results are assuming we have an infinitely rigid bullet that won’t deform on impact. Such is dumbing-down the laws of physics. For reference, aramid fiber (Kelvarⓒ-brand Kevlar™) has an ultimate tensile strength of 3.6GPa, level IIIA armor is 1.97mm thick. The 5.45, .22TCM, and 5.56 all penetrate, while the .357 and 9mm do not. This reflects real life, so yay!

The overarching effect of this change is that level II armor will stop maybe a 9mm round (assuming non-steel core or whatever), .22LR, and so on. IIIA will stop most pistol rounds, and rarely rifle rounds depending on round. IV will definitely stop rifle rounds. Maybe. We haven’t gotten to ceramics or steel plate yet.