Warbirds Gunnery Model

In this write-up I plan to talk about the Warbirds Gunnery Model, as far as I understand it, and how it relates to "real" gunnery. In order to become an effective shooter in Warbirds, one must understand the dynamics of the gunnery system in place in order to be most effective.

The purpose of a fighter is to disable or destroy enemy fighters. To do this a fighter needs to employ weapons, and the primary weapon used for Air-to-Air combat in WWII is the gun. All sorts of guns were used, from machine guns to cannon, from 7.7mm slug throwers to 75mm field howitzers. All the combat maneuvering and fancy flying in the world will not down a plane, only a gun (or in modern times a missile) will down an enemy plane, unless one considers colliding with the enemy a viable method of downing planes! Thus the whole purpose of Air Combat Maneuvering is to give a "guns solution" or a high-probability shot in the shortest amount of time. Defensive Air Combat Maneuvering is the art of preventing the attacker from getting a quality "guns solution". I will not cover this aspect of combat in this write-up. Rather I plan to describe the guns used in WWII aerial combat and how Warbirds simulates them.

First I'm going to describe my understanding of how "real" guns and gunnery on fighter planes of WWII work. To do that I need to define a few terms and concepts that relate to Guns in general and which I plan to use during this discussion.

• Caliber - This represents the size of the round that is fired from the gun. Caliber represents the diameter of the round as you look at it from in front of it. Some modern weapons have sub-caliber rounds to aid in muzzle velocity and armor penetration, but this generally was not used with WWII plane-based weapons. Note that Caliber does not represent how long the round is, for example the 30mm shell fired by the Rheinmetall MK 103 Cannon is much longer than that fired by the Rheinmetall MK 108 30mm cannon. Caliber can be used to measure approximately how much damage it does, as larger caliber rounds tend to do much more damage to the target than smaller caliber guns.
• Cannon - A Rapid Fire gun similar to a Machine Gun. These weapons fire rounds that often explode, have incendiary charges built in (to start fires), or have very hard tips and cores to facilitate armor and engine block penetration. This differs from a standard slug fired from a machine gun in that much of the damage from the round is done after impact when the round detonates. The rounds are often called shells as they are often hollow, filled with the detonation material and other substances designed to do damage to the target when hit.
• Machine Gun - These weapons are Rapid Fire guns that fire solid slugs, as opposed to shells. The primary damage from these weapons is Kinetic Energy, as the mass of the rounds is often very low (a single .50cal round weighs about 1/10th of a pound. A single 37mm shell fired by the P39D weighs 1.34 pounds). Since Kinetic Energy is a function of weight times velocity squared, the further away the round strikes the target, the less damage it does since it is moving slower. Heavier caliber rounds with the same muzzle velocity naturally do more damage, usually proportional to the weight.
• Rate of Fire - This value indicates the number of shells fired during a period of time. Often found in rounds per minute or rounds per second. Naturally a higher Rate of Fire yields more round per second that can do damage, but on the flip side means the duration of fire is much lower. Also a higher Rate of Fire means fewer weapons need to be mounted to do the same damage. For example, the A6M2 carries two 20mm cannon each firing 7.2 rounds per second. The Ki84 carries two 20mm cannon each firing 12.8 rounds per second, thus the firepower is equivalent to about 3.6 20mm cannon found on the A6M2 Zero.
• Synchronization - If a weapon is mounded behind the propeller and must shoot through the propeller, then it is often synchronized with the propeller to guarantee that the rounds do not destroy a blade of the propeller inadvertently. Unfortunately this often results in a decrease in the Rate-of-Fire for the weapon, but the trade off in weapon position can often be worth it, as weapons mounted close to the pilot's gunsight often don't have as much of a problem with convergence.
• Convergence - Most planes come equipped with several guns, often mounted far apart on the plane's wings. This presents a problem, especially with machine guns, as if the guns fire straight ahead, then the rounds fired are likely to be scattered all over the target, or worse, part or all of the guns rounds will miss the target entirely. The optimal solution for guns is to have the entire plane's compliment of guns hitting the same point on the target plane, thus increasing the damage done to that part of the target's structure. This is done via angling the guns inwards so that the rounds all "converge" on the same point in space ahead of the plane. If the shooter can place his target on this point, then his effective firepower goes up dramatically, as more rounds hit the same potentially critical part of the plane, and thus are more likely to saw off a wing or a tail or other critical area as opposed to merely poking holes. Another form of convergence is angling the guns up a bit to counter shell drop so that the rounds pass through the horizontal line of the crosshairs at the convergence distance.
• Projectile Dispersion - Real guns don't fire rounds in the same line through space, round after round. Despite advanced manufacturing techniques, rifling of the barrels, and other effects, when a round fires, there is often some sideways motion imparted on it. This is actually advantageous, as it makes hitting a target somewhat easier, and increases the probability of a stray round hitting something critical (like the pilot). To get an idea of this effect, take an Fw 190A8 up for a spin, and fire the cannon. You will see the plane rocking back and forth and the shells forming a "spread" in front. This is similar to what happens to "real" guns firing. In essence, over time the rounds fired form a hazy cone, like a shotgun's pellets. Naturally this makes it easier to score hits, especially with high rate-of-fire guns, as you don't have to have pinpoint accuracy to score hits. Unfortunately the end of the cone gets bigger the farther away the target is, and thus the effective concentration of rounds needed to down a plane is not possible beyond a point, as fire becomes a "peppering" effect. This is where cannon have an advantage over machine guns as most of their damage potential is explosive, thus a single shell or two that hits can do significant damage at long range.
• Shell drop - Gravity pulls everything down, as everyone knows. This is true for cannon/machine gun rounds as well. Given time, shells/bullets fired from a gun will fall at a certain rate, and this is the "Shell Drop". This is why you have to aim a little high when shooting a gun.
• Muzzle Velocity - This represents the speed of the round as it leaves the barrel (relative to the gun). This is essential for machine guns, and important for Cannon, and affects two critical areas of gunnery: Flight Time of the projectile (time it takes to get to the target), and Kinetic Energy. Naturally the shorter the flight time, the less lead you have to pull on the target and the sooner the target gets hit (and thus less time for a last-second maneuver causing a miss). Plus the higher Kinetic Energy translates into more damage on the target. In general guns with higher muzzle velocities are easier to hit targets with, can be used at longer ranges, and tend to do more damage.

In essence, guns are merely tubes that contain an explosion that propels a chunk of mass in a particular direction. WWII plane mounted guns were automatic firing weapons (for the most part) that fired either a solid slug, or an explosive shell. The action of a round firing would produce enough energy, in addition to propelling the projectile, to discharge the spent cartridge, load the next round and "cock" the gun for firing. This process is repeated to produce an "automatic" firing sequence and fire many rounds per second. "Charging" the gun preps it to fire the first round (usually by manually, electrically, or pneumatically cocking the gun and ejecting whatever is in the firing chamber), and can be used to eject dud rounds or help clear jams.

Once the projectile leaves the barrel, on whatever course it is headed once away from the barrel, the bullet is then subjected to a very strong wind force due to it's velocity plus any velocity the plane had, and is also subjected to the forces of gravity. Thus the projectile will slow down, and start to accelerate towards the ground, and often falls below the mach barrier in speed in less than two or three seconds (The flight path of sub-sonic bullets and shells gets real erratic when the Mach barrier is crossed). During it's flight the projectile will either hit another plane, or hit the ground. If it hits a plane, then the round will attempt to pass through the plane, unless stopped by something substantial like an armored plate or engine block. Naturally this will do damage to the plane and thus the whole reason for firing the round in the first place. If the projectile is a shell, then depending on it's design characteristics, it will explode, or it might delay explosion (to penetrate armor), then explode. A large percentage of a shell's damage potential is the explosion when it hits its target. A solid slug on the other hand depends on penetrating the plane and damaging critical areas via Kinetic Energy more than large scale physical damage caused by an explosive charge. This is why single shells tend to be far more damaging than solid slugs.

One effect of the extreme wind conditions is slip. If the plane's nose is not pointed directly into the relative wind (the airflow past the plane), in other words, the sideslip needle is not centered, then the relative wind will tend to push the projectile sideways when firing. This has the effect of making the rounds fly to the left or right of the center of the crosshair, depending on which way the plane is slipping and the magnitude of the slip. Naturally low velocity weapons are much more subjected to this effect. It is for this reason that most WWII gunnery manuals impress the need to check the sideslip indicator and correct any sideslip before firing the guns.

Real world gunnery is the art of delivering the most damage to the target airplane or ground target possible. In general, cannon are more lethal weapons due to their explosive shells and heavier rounds. Unfortunately cannon are considerably heavier and thus shoot fewer rounds per second for the same weight of machine guns (one cannon often weighs as much as several machine guns, depending on caliber). Plus cannon shells are considerably bigger than machine gun rounds, and thus the plane can not carry as many per gun and this inevitably leads to shorter firing times. Thus while cannon do more damage per hit, you have fewer potential hits than if you replaced that cannon with several machine guns, and further, you cannot shoot as long. This is why many WWII planes came with both machine guns and cannon, you use the machine guns to find the range and score hits (and thus scare or possibly kill the pilot, or puncture a fuel tank) and use the cannon to deal out the death blows. This is also where the type of planes that the shooter was going against often dictated gun type, for example the Luftwaffe were heavy on cannon due to their focus on shooting down bombers, as machine guns were not nearly as effective. The Americans in the Pacific, on the other hand, were going against rather fragile Japanese fighters, therefore the stronger punch of the cannon was not necessary, and the longer firing time and higher number of rounds in the air that machine guns give you made hitting the agile Japanese fighters easier while still doing critical damage when hitting. Unfortunately this aspect is not well modeled in Warbirds, as I will show below.

Warbird's Gunnery Model attempts to provide a realistic gunnery model, while minimizing the impact on the Frame Rate of the simulation, and the bandwidth between the host and a Front End. Naturally, the more rounds the Front End has to keep track of, the more CPU time is needed, and, depending on the implementation, the more bandwidth is needed to tell the host what parts of the target got hit and how much damage each part took. Warbirds addresses this problem by limiting the number of rounds per second it tracks. Basically there is an artificial cap of eight "tracers" for the Primary Bank, and eight "tracers" for the Secondary Bank. These tracers are then further divided up by the number and type of guns in each bank. Therefore, a Spitfire with two cannon in the secondary bank and four machine guns in the primary bank distributes four tracer rounds per second available to each cannon, and two to each machine gun. It is these tracers, not actual bullets, that are modeled by Warbirds' Front End when you fire a plane's guns. It is also these tracers that your ammunition counter shows when you fire your gun. For example, if your cannon loadout is 86, that means you have 86 1/8th second "tracers" left, which might represent 200+ rounds of "real" ammunition.

Each tracer follows the historical muzzle velocity, trajectory, shell drop, and convergence setting that a single round fired from that gun would follow. Warbirds models each tracer's physics as precisely it can, so that the tracer follows the flight path it would if it were a real round (except for dispersion, or projectile scatter). To model damage, the number of rounds that gun would fire per second is divided by the number of tracers that gun has available, and this is the "firepower" or "punch" that tracer has. For example, a FW 190A8's outer MG151/20 20mm cannon would have 2 tracer rounds each. These guns fire about 12 rounds per second each, thus each tracer represents 6 20mm rounds. A Bf 109K-4 has a single 30mm cannon in the spinner for the second bank, therefore has eight tracer rounds assigned to it. As that 30mm cannon fires 9.8 rounds per second, the number of rounds modeled per tracer is 1.2 30mm rounds each. In my plane charts, this value is shown under the category "WB Punch" as a number of rounds per "ping" or "tracer".

Now when a tracer hits something, that location receives the full brunt of the number of rounds the tracer represented. So a 20mm cannon with 6 rounds per tracer would model 6 20mm hits on a location per ping, while a 20mm cannon with 2 rounds per tracer would model only 2 20mm hits on that same location.

As real rounds have dispersion cones, such as a shotgun, and do not fire in laser-like lines, this is naturally a problem with Warbirds' gunnery model, realism. In order to approximate this, each tracer round is modelled as a 3 ft diameter ball, where the closest part of a target plane to that ball as the round goes by (if the part is within the ball) gets hit. The damage the tracer does is reduced to represent the fact that not all the rounds would really hit except in rare cases. This is why you can fire just above a wing and still hit with every round. A real gunnery model would have just those rounds that dispersed downwards hitting the wing, the rest sailing by harmlessly. One solution, of course, is to have this ball increase in size the longer the shell is in the air, and reduce the damage potential accordingly.

Unfortunately the 16 tracer limit effectively eliminates the rate-of-fire aspect of gunnery. A Spitfire Mk I has eight .303 machine guns, each firing 15 rounds per second for a total rounds per second for the plane of 120 rounds. A Bf 109K4 has one 30mm cannon and two synchronized 13mm machine guns for a total of 34 rounds per second. Each has the *same* number of tracers in Warbirds. This is important to remember, because in real life, the Spitfire would have a much closer round spacing than the 109, and thus would be much easier to hit with in snapshot situations as there are simply more rounds in the air in the same space to collide with the target. Also, planes with fewer guns per bank tend to appear to be easier to hit with, for the same reason, each gun fires more "tracers" per second, and thus the tracer spacing is much closer together and thus is much easier to follow the stream of rounds to correct your aim. This is why Focke-Wulf pilots often feel it's easier to hit with the Dora's cannon than with the Fw 190A4's inner guns (when the outer ones run out), simply because there are more "tracer" rounds assigned to those guns, despite the fact each "tracer" does less damage.

There are several situations that guns may be employed in air to air combat. I will cover these areas, and describe how real guns are in these situations and how Warbirds Gunnery model deals with these situations.

• Direct 6 shots: This is the most common type of shot in a surprise or chase situation. There are two categories of 6 shots, close-range and long range. Close range shots are where you are directly behind the target, long range shots place you considerably further away. Historically these were difficult shots to take as the profile of the target was often just a sliver. Projectile dispersion meant that unless you did the "Hartmann solution" and were so close your propeller could start shredding the tail of the target, most of the rounds you fire are going to miss even with perfect aim as shells scatter above and below the target. This naturally gets worse the farther behind you are. This is not accurately modeled in Warbirds, because the 3ft diameter "ball" effect of the rounds effectively increases the height of the wings and fuselage by 3 feet (3 feet on each side). For example a wing might only be 6 inches in size, but with the 3ft ball effect, that same wing is effectively 3 ft 6 inches in "height" that the center of the tracer has as a target area. And when one of the tracer rounds hit, *all* the rounds hit that location.
• Direct 12 shots: Often called Head-On shots. This type of shot has the same problem as 6 shots due to small target profile. This is exasperated by the fact that you are closing rapidly on the target and thus have a very short firing window. Historically these were luck shots, as it was extremely difficult to land hits with all but the biggest targets, and even harder to do any significant damage. Warbirds does not model this accurately due to the 3 foot inflation described above, and the fact that a glancing blow counts as a full tracer hit, thus causing the target to attract a lot more rounds than they should (giving a HO advantage to planes with high damage/ping values).
• Deflection Tracking Shots: This is when the target is moving at an angle relative to you and you have to fire ahead of the target to score hits. The tracking part is where you can hold this lead for a period of time, and correct your aim. These are historically the easiest shots to take as you have time to adjust your lead as you fire bursts and range in the target. This type of shot is modeled well by Warbirds. Unfortunately the target inflation caused by the 3ft diameter tracer model effectively multiplies the planform size and has nasty repercussions, depending on the plane type. For example, the P38 has a large hole in front of the horizontal stabilizer, but the 3ft diameter tracer model effectively seals this hole, making scoring hits (especially on the tail) easier (ask any 38 pilot). Planes with tail sections close to the wings suffer as well, for example the Ki84 tends to attract shells to it's horizontal stabilizer a large percentage of the time, as the tail section is only about 12 feet back, leaving a 9ft space for shells to miss with. Planes such as the F4U that have the tail well back from the wings don't suffer this problem as much (the F4U's tail is about 14-18 ft back, especially in deflection situations, thus giving a 11ft-15ft hole where tracers can pass through harmlessly).
• Deflection Snapshots: This is another area Warbirds does not model well. These are the passing-plane type shots, the shots where the target moves quickly through your bullet stream. The probability of scoring hits is directly proportional to the round spacing and inversely proportional to the speed of the target passing through the projectile stream, thus high rate-of-fire guns are far more likely to score hits. Warbirds does not model different rates of fire, thus round-spacing differences are not modeled at all. Plus, high movement through the bullet stream means that most of the time any hits that do land are most likely not going to do serious damage, as the average location hit will be hit by one, maybe two rounds, with damage spread across the plane rather than concentrated in one location. Since Warbirds models fixed rounds per tracer, any time a hit is scored, full damage is done to the hit section. Thus a ping from the outer guns of a FW190A8 on a target going by at 500mph at 90 degree deflection is going to cause 6 shells to hit that location. An elevator or stabilizer that can normally take a shell or two before failing is going to fail unrealistically. Historically snapshots were easier to score hits with, but much less damaging on the average than snapshots in Warbirds.

To sum up, Warbird's gunnery model would benefit greatly from a model-every-round gunnery model. Gun shot situations would be modeled accurately, and the need for abstractions like the 3ft diameter tracer round would be unnecessary as a "random" dispersion of rounds would solve the dispersion problem by default. Currently the gunnery model makes shooting easier than historical, and allows far greater accuracy and damage in extreme situations such as long-range shooting and snapshots. I believe that a model-every-round gunnery model that reflects reality would greatly enhance the feeling of being there and would open up a new world of differences between planes to explore, as well as allow advanced gunnery techniques such as dispersion control, different gun direction techniques (where each gun was pointed in slightly different directions to aid in scoring hits), ammunition type selection (such as incendiary for zeke-busting, armor piercing for Corsair/P47 busting, staggering types of rounds such as tracer spacing, extra tracer rounds to tell you you're almost out of ammo), etc. Default settings can be given, with a "custom gunsight"-like customization for those who really want to tweak their planes to perfection.

Two changes to the current modeling system would greatly enhance the current gunnery model without having to resort to a model every round system, or a shell scatter implementation, and would require far less implementation time. These are: Model the number of tracers per gun based on the rate of fire of the gun. Thus a SpitI might have 24 tracers for it's guns, while a 109K4 would have about 7 (assuming a tracer for every 6 rounds). This should be user selectable so that high end systems can have the advantage and realism of near-model-every-round, while those who do snapshooting and/or don't have the CPU horsepower can have a high rounds/tracer count. This would solve the different shell spacing issue with the different planes and guns. The other change is a variable width tracer ball. Have the tracer ball start out at 1 inch from the barrel, and grow to say 10 ft in size at 1000 yards. At the same time, the larger the ball is, the less damage a tracer hit causes. This simulates the "cone" of fire from a gun without modeling every round, and having to model projectile dispersion. Nothing would beat a modeling system that modeled every round, though, especially if it include projectile dispersion. Then Warbirds can add the Every Round Gunnery Model to its portfolio of 1sts in the industry!

Back To Main Page