On the 22nd March 2021, the British Government unveiled their plans to upgrade 148 of their
Challenger 2 Main battle tanks and designate them Challenger 3.
Challenger 3 has a host of improvements over it’s predecessor, but perhaps the most significant,
or at least the most discussed, is the decision to replace the 120mm rifled gun seen on Challenger
2 with a 120mm smoothbore.
For anyone with any knowledge of firearms, this seems an odd decision.
Almost every firearm made in the last 200 years has used a rifled barrel, that’s even
where the term “rifle” comes from.
But what if I told you that the vast majority of modern tanks already use smoothbore guns?
In fact, Challenger 2 was one of only a handful of vehicles still using a rifled barrel.
So what’s the difference?
What’s wrong with rifled barrels, what’s so great about smoothbores, and why are tanks
using technology that was considered obsolete in the late 1800s?
When tanks first rose to prominence in the First and Second World Wars, all of them used
rifled guns.
This means that there were spiral grooves cut into the inside surface of the barrel.
When the projectile is fired from the gun, it engages with these grooves as it passes
down the barrel, imparting a spin – this is known as spin-stabilisation, as the rotation
of the round means it’s much less likely to tumble or veer off course.
Firing the same round from a smoothbore gun, you would notice a significant drop in accuracy,
as the non-spinning round would be much more susceptible to tumbling and twisting during
it’s flight.
So, for any sort of solid round, rifled guns were just superior – the huge increase in
accuracy meant that you’d be a fool to try and use a smoothbore cannon.
In 1961 though, the Soviets revealed their new tank, the T-62, armed with a 115mm smoothbore gun.
Clearly something had changed.
And it comes down to ammunition.
A few years earlier, all vehicles were using some form of AP or Armour Piercing ammunition.
This is basically just a big bullet – fire it at someone and it will rip through the
armour and cause a lot of damage inside, especially if it has High Explosive filler in the case
of APHE.
And this worked fine for a while.
But, by the end of the war, armour had improved significantly, and the calibre of gun needed
to punch an AP shell through, say, a Tiger II or an IS-3, was massive.
And this was solved in two ways.
The first method was something called APDS ammunition – or Armour Piercing Discarding
Sabot.
The way this worked was to use a small , narrow penetrator made of a dense metal like tungsten.
Smaller projectile means more speed, which means more penetration.
But it was narrower than the tank barrel it was being fired from, so it needed another
component – the sabot.
The sabot would surround the penetrator, forming a seal with the barrel and the rifling.
Then, when the round left the end of the barrel, the sabot would fall away, or discard, allowing
the penetrator to fly towards it’s target at insane speeds.
The other method was to use HEAT shells – High Explosive, Anti-Tank.
These rounds are fired like regular AP shells but when they hit their target they detonate,
with the shaped charge inside forming the explosion into a very narrow, hypersonic jet
capable of piercing huge amounts of amour at any range.
This type of warhead was used on the bazooka and panzer Faust weapons, and became the go-to
method of defeating armour in the years following WW2.
But rifled guns actually limited the effectiveness of both of these ammunition types.
In the case of sabot rounds like APDS, the longer and thinner the penetrator was, the
more penetration it had, but it also required a much faster spin to stay stable, and as
you approached a length/width ratio of about 6 or 7, the rate of spin required to stabilise
these dart-like rounds was just too fast to be realistic.
HEAT rounds actually had the opposite issue – they were spinning too fast.
The effectiveness of the penetrating jet was diminished by the forces imparted on the jet
by the spin.
The shaped charge was trying to focus the explosion to one small point while the spinning
forces were trying to spread it out.
And, for a while, this was just sort of accepted.
The penetration of NATO guns was still more than enough to engage soviet MBTs of the time.
But, from a Soviet perspective, their current rifled guns would struggle against vehicles
like the M60 or Centurion, and a radical change was needed.
To defeat these vehicles, the Soviets would use a new type of round – Armour Piercing
Fin-Stabilised Discarding Sabot.
These rounds worked very similarly to APDS, except they had fins at the rear of the penetrator.
This meant that they could have extremely high length to width ratios, and that spin
rate didn’t need to be fast as they were fin stabilised rather than spin stabilised.
In fact, any sort of spin would actually be a detriment to these rounds, as the fins would
act like a sort of paddle as it flew, greatly decreasing velocity and thus penetration.
But the APFSDS rounds on the soviets new T-62 didn’t have spin, remember – it was a
smoothbore gun.
Smoothbore has historically had one advantage over rifled – the round leaves the barrel
at a greater velocity.
Rifling does have a huge increase in accuracy but some of the kinetic energy of the round
is used to spin it, so the shell does actually fly slightly slower.
Considering that APFSDS rounds already have very good accuracy thanks to the fins, this
boost to muzzle velocity, and therefore penetration made the Soviet smoothbore guns incredibly
scary.
The years following the introduction of the T-62 would see wide-spread adoption of composite
armour, necessitating the use of APFSDS worldwide.
The Leopard 2 would be the first Western Tank to mount a smoothbore main gun in 1979, and
the vast majority of NATO countries would follow suit, firing HEAT and APFSDS rounds
to defeat enemy armoured threats.
Now you may be wondering why it’s taken until 2021 for the UK to adopt this seemingly
superior technology.
And the answer?
HESH.
HESH, or High Explosive Squash Head, were a British invention in WW2 and they grew very
fond of them.
These rounds work by slinging a large amount of high explosive at an enemy tank, squashing
it against the side of the vehicle, and then detonating it.
This doesn’t actually penetrate the tank like most other shells try to, but the explosion
flings a shockwave through the armour.
As this shockwave exits the armour on the other side it takes with it small chunks of
metal, known as spall.
This spall will fly around the interior of the vehicle, hitting critical components such
as ammunition, fuel, or even the poor crew inside.
The thing with HESH is that it performs better if it’s spinning, as the spin will spread
the explosive out over a larger surface area, which in turn will produce more spall and
thus more damage.
And the Brits liked HESH.
It works against tanks of course, but you could fire it at just about anything.
It has great performance against fortified positions and I’m sure I don’t need to
explain what a round like that will do against lightly armoured vehicles or infantry.
However, HESH works best on big plates of steel.
The widespread use of ceramic composite armour may have diminished its ability to create
spall.
And to add to this, the introduction of spall liners – Kevlar layers designed to catch
spall fragments, meant that HESH just didn’t have the performance it used to.
Other nations had committed to using their tanks mainly as anti-tank weapons – hence
the focus on anti-armour performance and the adoption of APFSDS.
Britain still liked to think that it’s Main battle tanks would also be able to support
infantry, and rifled guns are still a lot better in that regard as they can use HE and
HESH, so that may explain why it took so long for them to make the big switch.
A switch which may have been, in the end, more about standardisation with her allies
in NATO than purely performance based.
So, to recap.
Rifled guns are great if you are using rounds that perform well when spin-stabilised – AP,
APDS, HE, HESH.
Smoothbore guns are great if you are using rounds that perform well when fin-stabilised
– APFSDS, HEAT-FS.
Both types have their own advantages and disadvantages, but in the modern world, especially regarding
tank on tank warfare, it seems that smoothbore guns are the current king, and Challenger
3 is looking to be an incredibly capable platform.
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