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Shell Ballistics

I now hand over the keyboard to another of our amazing players – Panzergranate – for an insight into shell ballistics.

Fear Naught
The_Challenger

 


 

I've been fascinated by the science behind punching holes in armour since I was a tank gunner back in 1979, having watched a burst of 3 Rarden APDS shots travel out to an old target hulk and seeing one of them bounce off. They all hit the same area of the target but one was travelling slightly slower or was slightly flawed, etc. which caused it to bounce off when it arrived.

Unlike artillery shells, which are mainly about delivering as large a quantity of explosives as possible over a reasonable distance with some degree of accuracy, anti-armour shells and shot are seriously designed, fettled and tinkered with just to "tune" a tad more maximum penetration ability (thus improving the chances of a first hit penetration).

One sometimes sees pictures of tank crews polishing AP ammo before a battle. I've a picture that appears in several books of Soviet T-34/76D crewmen with BR-350A or BR-350B APHEBC rounds laid out carefully on a groundsheet, polishing each round prior to stocking the tank. The HE shells are heaped nearby

Gunners W D Lewis and F J Hall of 33/61 Heavy Regiment, Royal Artillery, at Vergato, Italy, cleaning mud from shells for an American-built 155mm 'Long Tom' gun.

 

The process between when the gunner presses his foot down on the trigger pedal and the target suffering a hit are as follows:

The primer cap fires, igniting the propellant's rear end, which causes it to accelerate as it burns and converts to super-high pressure gases. (This is why length calibres are taken for purposes of ballistics calculations from the breech face).

Here is where we encounter the first chaotic influence that has a major effect on muzzle velocity: The propellant burn rate is not a constant but a variable with limits between not burning or smouldering to maximum efficiency.

The propellant, already travelling at transonic velocity, impacts the shell or shot base and imparts some of its inertia to it. Due to the heavier mass of the projectile, the propellant loses a lot of velocity in this impact. Both start to travel up the barrel at subsonic velocity but accelerating as the propellant continues to burn and form expanding gases.

If there is a tracer, it is ignited by this violent collision. In the case of APHE type shells (and some ordinary HE and frag shells) the violent collision starts the fuse-arming process.

Here is where we encounter the second and third chaotic influences that have a major effect on muzzle velocity: Projectile Gas Band Sealing and Muzzle Residue.

The copper rings on the base of the shell or shot act like piston rings. However, they are not precision-fitted due to the manufacturing process, and fitment is akin to heat-forcing a metal tyre onto a wagon wheel. This means these copper sealing rings grip the barrel and the rifling, varying from loose through optimum to too tight. Some tank crews would measure, grade, file and adjust copper drive bands on the rounds sometimes just to gain a slight improvement.

Too loose and a lot of the propelling gases escape around the shell or shot, resulting in a reduction in muzzle velocity. Too tight and the round has a reduction in velocity due to excess friction in the barrel.

All rounds, whether they are AP, HE, Smoke or otherwise, that have exceptionally tight drive band fit in relation to the barrel rifling (high friction losses) are usually set aside as "scouring" or "muzzle clearing" rounds.

The other problem is muzzle residue. This is left over from the previous firing of the preceding round. If the last round didn't burn cleanly, it will leave a lot of soot and residue in the barrel. This will cause a resistance to the next projectile to travel up the barrel. However, firing a projectile which is known to have very tight drive bands will scour this mess out of the barrel and clear it. If you've ever heard historical accounts of a gunner firing a HE, smoke or some other round that would be pretty useless against an AFV, before following up with an AP round? Now you know why.

M4A3E8 "Sherman" Tank of Company B, 72nd Tank Battalion, 2nd Infantry Division fires its 76mm gun at enemy bunkers on "Napalm Ridge", in support of the 8th ROK Division. Photograph is dated 11 May 1952. Photograph from the Army Signal Corps Collection in the U.S. National Archives. Photo #: SC 398704. 

 

So after all this, the shell or shot exits the muzzle with whatever velocity it has gained during its acceleration down the barrel. Any unburnt propellant and explosive gases also exit the muzzle, causing recoil, a fireball and smoke.

At this point, the shell or shot is in flight on course for the target. Since leaving the muzzle, it has started to decelerate due to the passage through the atmosphere. The actual amount of drag on the shell or shot depends on the time of the year, the weather and moisture content of the air. A hot humid summer day generates denser moisture laden air which imparts more aerodynamic drag than a dry sub-zero freezing winter day.

On a normal spring to summer day, an average APHE shell or AP shot will lose around 11% of its penetration power by the time it has travelled 500 metres, when compared to its maximum rating at 100 metres. This means that if it could penetrate up to 100mm at 100 metres, it can only penetrate up to 89mm after 500 metres.

If the shot is APC (no ballistic cap) then the aerodynamic drag will be worse.

If the shell or shot is of low mass (APCR) or gyrates as it becomes unstable (APDS) with range, then it will also suffer more aerodynamic drag.

Panther tank mantlet penetrated by a 90mm M3 firing HVAP shell.

Panther tank glacis penetrated by a 90mm M3 firing HVAP shell.

 

Another factor influencing the shell or shot's accuracy is the varying density of the air along the flight path. We experience these as breezes and wind. These pockets of varying air pressure can alter the aim minutely over longer distances and this will affect low velocity guns the worst. Higher velocity projectiles are less affected.

Our shell or shot reaches the target and strikes the armour. If it is a plain APHE shell or AP shot the point of contact will superheat and melt as the velocity of the projectile is converted into impact pressure.

Here we encounter more chaotic influences that will determine penetration or not.

Assuming that the target's armour is well within the limits of the projectile’s abilities, the shell or shot suffers a massive vibrational shock to its structure on impact, which will seek out any casting flaws and could shatter the projectile. This is classed as a fail.


To prevent this from happening, the manufacturers face-harden the ammo's surface. Ammunition designers fit a mild steel or steel cap (which has a low melting temperature) to the shell or shot.  This will melt on impact, softening the shock felt by the shell or shot, and if the armour is not square onto the firing gun, it will cause the shell or shot to grip at the nose end and the tail end to pitch slightly, reducing the angle of incidence. If the angle is too great, the shell or shot will plough across the armour, losing energy and performance to friction until it can either dig in and begin to penetrate or be deflected away from the armour.

Let’s assume that our shell or shot was successful in penetrating.


Panther tank lower front plate penetrated by a 90mm M3 firing AP shot.

The penetration process sees the superheated shell or shot melt through the armour using focused pressure through inertia, ejecting molten armour rearwards as it passes through.

On entering the target's interior, the shell or shot could be anywhere from dull red hot to white hot, depending on the velocity, mass and energy plus thickness of armour traversed. In the latter case (superheated shot) the crew are instantly killed, any ammunition spontaneously explodes, and if it is an M-4 Sherman, the aviation fuel-filled gas tank boils and explodes in a massive fireball. That's in real life, not in WoT! However, only tungsten, tungsten-cobalt and uranium solid shot will superheat to white hot after penetration. Forged steel, forged iron and cast iron shells or shot would liquefy at those temperatures, causing a failure to penetrate the armour.

 

Soviet 76mm BR-350B

German 8.8 cm PzGr.39/43

 

Usually though, the shell or shot is just red hot, unless the armour was extremely thin, in which case shattered fragments of the armour, rivets, bolts and anything on the inside face of the hit will spray around the inside of the target at non-lethal but hazardous velocities.

If the round is solid shot it will ignite any flammable object it contacts, especially ammunition, fuel, oil, etc.

It also carries more mass for its size than an APHE shell so it can smash its way through equipment, engine blocks and so on.

If the round was an APHE type shell, then this explodes inside the target just as it enters, in mid-air or after bouncing around for a few seconds. If the contents are explosive, then the crew will most likely be killed by what is the equivalent of a mortar bomb HE sized explosion inside the target. The enclosed nature of the explosion serves to amplify it. Any ammunition or machinery reached by the blast's flame front will also be affected. With ammo, a chain reaction will ensue.

If the content is explosive or incendiary (BR-251B, BR-350B, etc.) then the crew will be sprayed with burning gel, as will any ammunition and equipment, effectively putting the target and crew out of the fight.

On the German PzGr.39. APCBC-HE, the projectile carries a base-mounted grenade charge containing a few grams of explosive. This causes a shrapnel effect inside a target, as the primary damage infliction is performed by the shot / shell it is attached to. The grenade serves as a backup damage infliction weapon.

And that is basically what happens without too much science.

 

Panzergranate

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