Weapons and Ammo

Weapons are the reason for existance of fighters.  Bombers provide a means to deliver bombs to ground targets.  Fighters are necessary to shoot down bombers.  Fighters were also needed to protect bombers from other fighters. In the pre-missile days of WW2, guns were needed to shoot down fighters, as the only alternative is harsh language, which is real hard to hear over the din of the engine.

In a World War II aircraft simulation, accuracy of the aircraft weapons is  paramount.  To that end this involves modeling every round, ensuring accurate trajectories, using accurate ammo sequencing (belting), and providing an accurate collision detection and terminal ballistics system.

Accurate trajectories required a high-quality bullet position and velocity model.  To accomplish this, WW2OL uses the G1 ballistics formula, which accurately models the drag applied to a bullet during flight.  A simple drag formula can be written as:

Drag = 1/2 * density * v^2 * BC (where BC is the ballistic coefficient). 

The problem with this method is that drag rises sharply as rounds fly supersonic, then returns to a (higher) velocity squared curve at higher supersonic airspeeds.  The G1 model uses the following formula:

Drag = p/p0 * G1(v)/BC * v

(where p = air density at current altitude, p0 = sea level density, G1 is a speed-dependant constant that models the variations in drag due to mach number, and v is the speed of the round at that instant)

With a G1 good look-up function, one can model the drag accurately, therefore the trajectory.  There are many tables available that provide the appropriate constant for a particular speed.  WW2OL uses a modified version of the above formula that works on mach-speed instead of velocity, since the drag variations (from the standard velocity squared curve) are mach-related, and mach speed changes with altitude (actually with temperature, which changes with altitude).  WW2OL uses a 1974 NASA standard atmospheric model, which provides both density and temperature values vs altitude, from which mach speed can be attained.

In addition, gravity forces are applied to all ammunition.  This has a marginal effect due to the high speed and drag forces, but is enough to cause ammunition to miss a target if it isn't taken into account.

This provides a very accurate trajectory modelling system.

Ramifications of this system are:

• At higher altitudes, rounds will slow down less
• Rounds fired will leave the muzzle at their muzzle velocity plus the velocity of the gun (this includes aircraft speed plus rotational effects, so if you roll your plane fast, wing guns' rotation will affect their trajectory)
• At higher aircraft speeds, rounds will slow down faster relative to the aircraft
• Different rounds have different ballistic coefficients, and will slow down at different rates
• Gravity effects are entirely dependant on the direction of flight (if you shoot straight up or down, the round will not "appear" to drop due to gravity, even though gravity is in effect)
• Rounds fired at an angle relative to the airflow will be deflected (this is most noticeable when firing defensive guns on bombers)

In WW2OL, as in real life, guns are set to converge at some set distance.  This is controlled (pre-flight) by the ".conv" command in-game.  All the guns will put the center of their bullet patterns at a point in space a set distance in front of the gunsight.  Note that this is a static setting, and only "works" when sitting on the ground, with the plane held level.  The general effect is that guns will pass through the gunsight center at close to the specified range if you fly straight/level.

Dispersion is modeled in WW2OL.  This makes guns fire in a "shotgun" pattern, in a sort of cone from the muzzle.  Rifling was an important invention for gun accuracy when it was used on guns, but it isn't perfect.  In addition, gun recoil, especially on a flexible structure like an airplane wing, can cause subsequent rounds to vary in velocity direction.  This can cause significant spreading of ammunition at further distances.

In addition, different rounds of the same type can have slightly different muzzle velocities (no two rounds are perfect, and when you mass produce millions of rounds of ammunition, you are not going to get match-grade ballistic properties, instead different rounds will have slightly different muzzle velocities).

Some weapons are mounted so that they shoot through the propeller disc.  These guns carry synchronization equipment to ensure that the gun does not fire when it would hit the propeller.  This reduces the rate of fire by a certain amount (usually 15% or so) vs an unrestricted weapon.

Aircraft weapons typically fired a varied set of ammunition types in specific sequences.  All the aircraft in WW2OL carried tracer ammunition, either in an AP configuration, or an HE configuration.  Thus a gun might fire an HE round followed by an AP round followed by a tracer round, for example.  In addition, in order to minimize "grouping" of ammunition types when more than one gun was fired, each weapon had a different stagger to the belting.  The first round out of the left gun might be an HE round while the first out of the right gun might be a tracer round.

Terminal Ballistics describe what happens when the ammunition hits something.  In the case of vehicles, when a round hits a vehicle, several things can happen.  All rounds have a penetration constant, which is used to determine if the round can punch through the object being struck, based on impact angle, and resistance of the object being struck.  An engine block or piece of armor is generally going to have a much higher resistance than airplane skinning, or even a human body.  Typically, armor piercing rounds will punch through much tougher stuff than high-explosive rounds.  Larger calibre rounds typically have better penetration properties for a given velocity, but penetration is very dependant on striking velocity.

The damage applied to a component depends on the nature of the hit.  The damage potential is based on the kinetic energy lost by striking the component, plus any HE effects of a round going off nearby (including shrapnel).  Component type matters too, as an armor plate will pretty much ignore damage (it's a hunk of metal that takes a huge amount of energy to significantly degrade), while aircraft skinning can degrade aerodynamics, holes in radiators and fuel tanks can cause leaks, fuel hits can cause fires/explosions, ammo/weapon hits can cause jams/ammo explosions,  hits to engines can degrade/kill the engine, and crewmembers can die when hit (wounded crewmembers lose a lot of their strength, making aircraft maneuvers difficult).

High explosive rounds are typically found on 20mm and larger rounds.  This is an explosive filler with a metal shell, whose purpose is to both damage via concussion (the expanding gas), and the fragments thrown out.  Typically a fuze will be activated, which will run for a short while (such as a millisecond or less), then burst the round.  Damage can be done via both the concussive blast and fragments.  Typically HE rounds can inflict more damage than solid rounds, but the fuzing mechanism can be problematic, because if the fuse waits too long (such as when striking a wing), the round might have left the vehicle before bursting.  In addition, if the round bursts too soon, it might not reach a critical component (such as a pilot when hitting the rear fuselage of a fighter).

Typically HE rounds will self-destruct after a certain flight-time.  This is primarily a safety device, to reduce the risk of unexploded ammunition ending up being littered around the countryside.

Armor piercing ammunition prioritizes penetration over damage.  There are many components onboard an aircraft that are vulnerable to penetration, like fuel tanks, engines, crew, ammunition, etc.  Rather than inflicting damage on a large scale like high-explosive rounds do, AP rounds try to punch clear through the aircraft, hopefully through something critical.  Many more rounds are necessary to cause structural failure than with HE rounds, but AP rounds are much more likely to cause a critical-system hit.  This is why AP-centric weapons packages typically fire many more rounds than HE packages.  The idea is to throw as many AP rounds into the enemy aircraft to dramatically increase the probability of scoring a critical hit.

Specific Weapon Descriptions

As the war went on, heavier and heavier weapons were employed.  At the start of the war, .30cal weapons were the norm, with some nations putting up to twelve guns onboard their fighters.  Most nations jumped to 20mm cannon (and later 30mm in the case of Germany), except for the US, who found that banks of .50cal machineguns were more than adequate for their aircraft.

Note:  Ammunition velocity and penetration data are for the gun fired horizontally on the ground while not moving.  Penetration data is for a 0 degree hit.  Real world penetration is likely to vary based on striking angle, and differences in striking velocity.  In addition, muzzle data is determined via firing the gun and recording the impact velocity at specific ranges.  Several rounds are fired and the average result is used.  Due to the nature of the WW2OL gunnery model (which models muzzle velocity variation), like ammunition types will typically have slightly different velocities.  Penetration data is calculated from the penetration constant and the WW2OL armor penetration formula, rounded to the nearest mm, and is equivalent to how much armor the ammunition type could penetrate.  Note that aircraft have fuselage skinning (at high obliquity) and other components that can slow down ammunition considerably before pilot armor is hit, so be careful when determining if a particular round could hit the pilot from a direct-6 shot.

.30cal class guns

.30cal class of guns fire rifle-class bullets (basically the same sort of ammunition a rifleman would fire with his gun, should he get AP or Tracer ammo that is.  A low punch weapon with not that much kinetic energy (compared to the heavier weapons), this class of weapon relies upon penetration of light structure and critical hits to down aircraft.  The rounds are small and light, and thus tend to slow down quickly meaning that long range hits are harder than the heavier weapons, and are typically impotent.  Most .30cal-only airplanes carry a lot of these guns, and are designed to get in close and hurl literally hundreds of rounds at a target.  Pilot armor and self-sealing fuel tanks made these weapons less effective, and all the nations went to heavier weapons later on as a result.

7.92mm Rheinmetall MG 17

This is the standard belt-fed German light machinegun for their fighters until replaced by the 13mm Rheinmetall MG 131 later in the war.  It is a good weapon, with a high rate of fire, but like all light machineguns it won't do much damage except on a lucky hit or an en-masse strike.  A variant of this weapon, using a drum feed and a lower rate of fire was used as a flexible mount for defensive fire.

Mass: 12.6kg

Rate of Fire (in WW2OL): 15.5/s (synchronized), 18.3/s (non-synchronized)

Ammo:

Typical belting:  ap, ap, ap, ap, ap, tracer

7.62mm/7.7mm Browning MG (.30/.303 calibre)

The Browning .30cal machinegun was the primary .30cal machinegun for the American military during WW2, used extensively by the Army both in its aircraft and on the ground.  The British used a .30cal chambered version of this gun many of their aircraft, and the Browning also sees service in WW2OL in the French Air Force on aircraft imported from the US.  The US eventually upgraded their air force to be primarily .50cal Browning equipped later in the war to give their aircraft more punch.

Mass: 10kg

Rate of Fire (in WW2OL): 19.0/s

Ammo:

Typical belting (.30cal): ap, ap, ap, ap, ap, tracer

Typical belting (.303cal): ap, ap, ap, ap, tracer

7.5mm MAC 1934 MG

This is the French standard light machinegun found in fixed mounts on the later French aircraft.  It has a lighter round than the other nation's .30cal class weapons, but given the nature of .30cal class hits on airplanes, this isn't that big a drawback.

Mass: 8.5kg

Rate of Fire (in WW2OL): 16.6/s

Ammo:

Typical belting: ap, ap, ap, ap, tracer

.50cal class guns

These guns are bigger caliber machineguns.  The principal is the same, but the difference is that the ammunition is larger, and heavier.  What this means is that they will go through much tougher stuff (like pilot armor), and will not lose their speed as quickly (meaning that longer range shots are both easier and more effective).  .50cal guns start to carry enough punch where serious structural damage is possible without requiring ridiculous numbers of hits.  However, .50cal-only aircraft still carried lots of these guns to maximize their effect.

12.7mm Browning MG (.50 calibre)

This is the beefed-up big brother of the .30cal Browning MG.  Also known as the Ma-Deuce (for M2, the weapon's designation), this is quite possibly the most famous machinegun in history.  Many people attribute legendary status to this weapon.  Still in use today by the US military, it is one of the best .50cal machineguns ever mass-produced, having a high muzzle velocity and good punch.  The only drawbacks are weight (higher veloctiy guns tend to be heavier), and lower rate of fire vs other nation's .50cal class weapons.  The Japanese air force copied the M2, and later made a 20mm cannon version out of it.  Later in the war, most US fighters carried up to eight of these weapons.  It was only when US fighters fought post-war jet aircraft and enemy bombers that the .50cal started to prove insufficient for modern air combat.

Mass: 29kg

Rate of Fire (in WW2OL): 12.4/s

Ammo:

Typical belting: ap, ap, ap, ap, tracer

20mm class guns

20mm ammunition is the smallest ammunition where you can put in a significant amount of explosive to create an HE round.  Some nations tried putting explosives into smaller rounds, but a 20mm is really the smallest calibre that can bring down a fighter with only a few hits.  Armor piercing ammunition was still used (and quite effective vs pilot armor, or ground attack), but most 20mm guns carried high explosive ammunition.  Typical HE rounds are fragment-based, carrying a small charge (typically 6g of TNT equivalent) intended to send fragments flying like a mini-grenade.  However, with aircraft structure, it turns out that the blast effect is more damaging.  Only the Germans really exploited this trait, creating what they called the "mine" round, containing 18g of TNT equivalent, and only a thin shell wall to contain the explosive.  This made the German 20mm significantly more potent than other nation's 20mm cannon, especially against bombers.  After the war, most nations adopted the concussion-based ammunition principal, and 20mm cannon are still in widespread use today.

The 20mm guns suffer the most from disparate ammunition types.  While the British and French guns had similar ballistics for their tracer/HE/AP rounds, the German's 20mm rounds were all over the map in terms of ballistics.  Part of the reason for this was the fuzing the Germans used, which required a flat nosed round, seriously degrading aerodynamic properties.  In addition, the lighter mine round would have a higher muzzle velocity and lower ballistic coefficient than the standard German HE round.  Since the Germans did not make a tracer version of their mine round, their guns would fire a mix of mine and HE/tracer rounds, along with the standard 1 out of every 5 AP rounds.

20mm Oerlikon MG FF Cannon

The Swiss Oerlikon class of guns were widely used in WW2, used by both the Axis and the Allies.  This gun (and its derivatvies) came in three variants, a low velocity light weapon meant for aircraft, a high velocity, low rate of fire used primarily for anti-aircraft work, and a middle-of-the-ground weapon.  The German aircraft weapon was called the MG FF, the Japanese called theirs the Type 99-1 and used it in the Zero fighter, and the US navy used the high-velocity version extensively in ship defence.

The German version is based on the  FF F version of the Swiss Oerlikon gun.  It is a lightweight gun (about the same weight as the US .50cal Browning MG), possesses good firepower (especially with the mine rounds), and decent rate of fire.  It's biggest drawback is its low muzzle velocity and its varied ammunition ballistics, making deflection shooting problematic.

Mass: 28kg

Rate of Fire (in WW2OL): 9.8/s

Typical belting: HE/tracer, mine, HE/tracer, mine, ap

20mm Mauser MG 151/20 Cannon

Arguably the best 20mm cannon of the war (before the rotary cannon was put into production near the end of the war).  Other weapons might have had a higher velocity, or a better rate of fire (but not both), but none had the combination this gun had and the mine round made this cannon one of the most lethal of the 20mm class.  This cannon became the primary German cannon for their aircraft after it was introduced, and the Fw190 eventually mounted four of these cannon (six with a special anti-bomber loadout).  This gun came in both unsynchronized and synchronized formats.

Mass: 42kg

Rate of Fire (in WW2OL): 10.0/s (synchronized), 12.3/s (unsynchronized)

Typical belting: HE/tracer, mine, HE/tracer, mine, ap

20mm Hispano-Suiza 404 Cannon

A French design that was exported to England and the US, this was an excellent 20mm cannon.  While the German MG 151/20 might have had more firepower, the Hispano-Suiza cannon had two advantages that helped in shooting targets.  The muzzle velocity is higher, and the ammunition is much more uniform in ballistics than the German weapon.  This is a long and heavy weapon, considerably heavier than the German 20mm cannon.  This is a direct result of the higher muzzle velocity, which required a sturdier gun and a longer barrel to achieve these results.  However, the results are very impressive.  Long range shooting is much easier in with the Hispano Suiza class of weapons, and the ballistics make hitting targets in deflection shots easier.  As an added bonus, the muzzle velocity and ballistics are similar to the Browning .50cal machinegun, meaning that the P38 and .50cal-equipped Spitfires do not have to worry about different trajectories between their cannon and their .50cals.  The HE rounds also carry a ballistic cap, which meant that they didn't suffer the same kind of drag problems that the German HE ammunition had.

In addition, the high muzzle velocity make this cannon very useful for ground attack work, as it will penetrate light armor easily (<20mm plates).  Keep in mind that except for the P39, the French 20mm was belted with only one AP-class round every 5 rounds.  The P39, however, was equipped with nothing but AP ammunition, thus making it possibly the best gun-based anti-armor airplane in WW2OL.

Mass: 60kg

Rate of Fire (in WW2OL): 12.0/s

Typical belting (D520, P38): HE, HE, HE, HE, AP/Tracer

Typical belting (P39): AP, AP, AP, AP, AP/Tracer

20mm Hispano-Suiza Mk II Cannon

This is the British copy of the French 404 cannon.  The Mk II was the standard 20mm cannon for a good part of the war, while later British aircraft used the improved Mk V version (higher RoF, lower weight).  A quad 20mm arrangement became the standard armament as the war progressed, replacing the banks of eight .303cal machineguns, which became increasingly less effective as time went on.  The British version fires at a lower rate of fire than the French gun, and is lighter, but is otherwise identical.

Mass: 50kg

Rate of Fire (in WW2OL): 9.9/s

Typical Belting: HE, AP/Tracer, AP

(some weapon/ammo data sourced from http://www.geocities.com/CapeCanaveral/Hangar/8217/fgun/fgun-pe.html )