You could argue that by the year 2400, cartridge-based combustion-propelled firearms will be horribly obsolete. Certainly many fictional futures have taken that route and opted for only rays guns, lasers, blasters, phasers, ion cannons, and other such options.
But, if you think about it, the sophisticated modern firearms of today are based on the exact same principles as weapons created in the fourteenth century. Using combustion to propel a small piece of metal very, very fast has proven to be a very effective way to harm living things. Firearms have been around for seven hundred years without becoming obsolete, so there's no reason why they wouldn't still be in use in some form four hundred years from now alongside whatever other new ways of killing get created.
When it comes down to it, however, it really doesn't matter how likely anything is in reality. From a gameplay perspective, we want to have many options for our players. We want them to be able to use different kinds of guns with different gameplay characteristics and to be able to upgrade those guns in numerous ways. Our goal is to add variety to the experience of playing the game, not accurately predict the future of weaponry.
As I started prototyping the gun mechanics in code, I found a lot of examples and tutorials scattered around the Internet about how to do guns in Unity. Most of those tutorials recommended a simple raycast (combined, of course, with sound and visual effects). You cast a ray out from the gun's barrel and see if it collides with something. If it does, you have a hit and the result of that hit happens immediately. And why not? As far as human senses are concerned, bullets from modern guns might as well be instantaneous except at the very longest range of the most powerful sniper rifles.
But that approach is no good for our game. One of the main benefits of the later advanced weapons in the game is that they don't suffer from some of the problems that ballistic firearms have. For example, when shooting at long range with a sniper rifle, you have to account for the trajectory of a bullet, the fact that gravity pulls the bullet downward as it moves forward, and the fact that other forces, like wind, can act on the bullet. Ray guns don't have that problem. They're simply point and shoot, so to speak, and raycasting makes perfect sense for those later, more advanced weapons. But raycasting doesn't take the realities of a physical world into account for a ballistic firearm.
I want the firearms in the game to "feel" real, and I want the bullets to behave the way a real bullet would. I'm all for cheating when it makes for a better experience and raycasting bullets is a great solution for many types of games, but the mechanics I've been working on really put the behavior of the guns front and center, so I really want the bullets to be part of the physics simulation.
I did find some tutorials and code examples that created bullets as rigidbody objects and applied force to them, which is the basic approach I wanted to use. There are some problems with this approach, however. First and foremost is simply that bullets travel very, very fast, and physics calculations only happen so many times a second. On mobile devices, those calculations tend to happen less times per second than on a desktop computer or console because there's simply less computing horsepower available. What can happen as a result, is that bullets can pass right through objects they should have hit. In one frame, the bullet is on the near side of the target, and by the time the next physics frame rolls around, the bullet is on the other side of it, and no collision is detected.
For a desktop game, this is easy to rectify; you just crank up the physics frame rate (which is distinct from the display framerate in Unity) so that the calculations happen more often. For a mobile game, that's not an ideal solution. You have to use the available CPU (and GPU) power efficiently on mobile if you want an overall experience to be good. Fortunately, there's a good solution to this problem on the Unity Wiki. You have your projectile do short ray casts in any physics frame where it travels far enough between frames to have missed a collision.
The bigger problem for me was trying to figure out just what values to use in the physics system. How much mass should the bullet have? How much force do we need to apply to that bullet?
The examples I found seem to have arrived at values by pure trial and error, and they all felt "off" to me. Many of the examples I found used the default mass value for the bullet, for example. In Unity, the default value of "1" is equivalent to 1 kilogram. If you've ever held a bullet, you know that it masses nowhere near a kilogram. Even the giant .50 caliber BMG round doesn't come close. You know what shoot bullets that weighs a kilogram? Battleships, not rifles.
Instead of taking the same trial-and-error approach to getting mass and force values that feel right, I decided I'd do a little research. There's a lot of science behind guns and a lot of people who are interested in guns, so I figured it couldn't be too hard to find real data on real bullets.
It ended up being even easier than I thought it would be. Wikipedia has gathered that data for pretty much every modern form of ammunition, including the exact mass of the bullet, the muzzle velocity, and the amount of energy used to propel the bullet to that velocity.
So, I gathered up that data for an assortment of assault, sniper, and high-powered hunting rifles in a spreadsheet. You can download that spreadsheet here, if you're interested.
Using the bullet's mass in Unity's physics system is easy enough. Just divide the grams by 1000 and that gives the value to use as the projectile's rigid body mass. But, how do we know how much force to apply to the bullet? Unity's documentation for the AddForce() method doesn't say what units it wants for input.
After digging around, I found that somebody had actually gone through the process of figuring out the answer to that while trying to counteract gravity for an object in their game. They determined that the AddForce() method uses 1/50th of a joule as its unit. Since we know how many joules of energy propel each of these modern bullets, we just multiply the number of joules by 50 and feed that value to the AddForce() method.
Great! But modern guns also spin the bullet as it travels down the barrel. In fact, the name "rifle" comes from the grooves in the barrel that cause that spin. After experimenting a bit, I came to the conclusion that for purposes of the game physics, rifling really isn't needed. Rifling helps deal with real world problem that just aren't present in the game's physics engine unless we add them.
But, I decided I still wanted the bullets to spin.
That may seem like an unnecessary bit of realism, but there's actually a reason for it. In some situations, like if you finish a level with a head shot, we're going to slow down time and follow the bullet to its target with the camera. It's a little clichéd, but it's still a cool effect when used sparingly. When we do it, though, I don't want people noticing that the bullet isn't spinning.
And they will.
In the real world, the twist rate of rifling is measured a couple of different ways, including revolutions per minute and the length required to complete one revolution inside the barrel. What it's not measured in is joules. And since this is just for show, I don't want to actually model the rifling into the gun's physics model, because that would be a lot of work and would force the physics engine to do an awful lot of calculations. Instead, I just want to spin the bullet right the moment it is spawned. Unity will let me do that in one line of code using the AddTorque() method. This method takes the same 1/50th of a joule input as AddForce().
But how much torque in joules should I add to the bullet's Z axis?
Honestly, I have no idea, and I really don't think it's worth spending a huge amount of time trying to figure it out since it doesn't actually affect the bullet's trajectory. I know it's a lot less force than is used to propel the bullet itself, so I'm going to start with a small number - 100 units (2 joules) - and see how it looks when we switch to the bullet cam. I'll then tweak the value if it doesn't look right. Sometimes trial and error is the right approach. Or maybe it's the lazy approach. Maybe it's both. Regardless, it's the approach I'm taking here.
I threw together a quick shooting gallery to test my real-world-based gun physics. Yes, it's an ugly shooting gallery. This is what you get when a developer throws something together quickly instead of asking his artists to make it for him. Despite the ugliness, I'm actually pretty darn happy with the results. Here's what it looks like shooting a gun based on values taken from the .300 Winchester ammunition:
There's still a lot of work to be done on my gun class. I need to get recoil in there, for example, as well as muzzle flash. But, I've got most of the basics down for building a variety of weapons by simply configuring parameters in Unity's inspector. Change the weight and force and a handful of other parameters and you get a gun that behaves and feels very differently. Change the 3D model as well, and you basically have a new, different gun.
On a related note, my early prototypes used the gyroscope for aiming on mobile devices. It had a really natural feel that I loved, but it proved problematic when you zoomed in very far. The tiniest movements from holding the device in your hands would translate into very noticeable, unwanted movement. That movement actually felt like actual shake and scope drift, but beyond about 4x magnification, the game became basically unplayable. I spent some time trying to add stabilization and smoothing to the gyroscope input, but was never happy with the result or the amount of control we had over it.
After a while I admitted defeat and ended up ripping out the gyroscope code and replacing it with code that used the accelerometer for aiming. I then added scope drift back in algorithmically and created a parameter for it. That means we can easily change how steady a gun is when used. A rifle with a bipod, for example, will have almost no drift, while a large gun used while standing will have a fair bit more.