Why are tank guns smoothbore?

A view from the breech of Rheinmetall's 130 mm L51 smoothbore gun.

Almost all modern tanks use smoothbore main guns, which can be somewhat confusing when everyone knows rifled guns are more accurate. Why has smoothbore become the only choice for modern large calibre guns?

Its all about ammunition

Modern firearms are all rifled, and most know that this is to create a gyroscopic effect that makes projectiles more accurate.

The rifling in a tank gun, as with a small calibre weapon, is there to stabilise the round in flight. As the round passes down the barrel, it engages with the rifling which spins it. This spinning creates a gyroscopic force that prevents the round from tumbling or altering from the axis upon which it was fired.

And this remains true for normal, bullet-like projectiles. But as tank ammunition became more bespoke and distinctly un-bullet like (perhaps not the most technical term there), things changed.

To understand this, a quick jaunt through tank ammunition history is in order.

AP/APC

Armour Piercing (AP) is to all intents just a large bullet. They usually have a toughened core and can have a high explosive fill too, but they are still quite basic in their shape and design.

A typical AP round like the British 2 pounder projectile weighed 900 g, had a velocity of 792 m/s and could penetrate 40 mm of armour at 1,000 m range.

However, even by the mid 1940s AP was being overmatched by armour and the practicalities of making an AP round that could penetrate contemporary tank armour was excessive. So, APC came along.

Armour Piercing Capped (APC) rounds, like the 75 mm M61 Armour Piercing Capped Ballistic Capped (APCBC) used in the M4 Sherman, fitted aerodynamic and penetrator caps onto the projectile, allowing a more effective penetration especially into sloped armour. The M61 projectile weighed 6.6 kg, had a velocity of 617 m/s and could penetrate 70 mm of armour at 1,000 m - almost double the performance of the earlier 2 pounder AP rounds.

The race of penetration versus armour protection never relents though, and soon enough the late war monsters like Tiger and Panther were becoming impervious to the capped rounds, which resulted in the production of APDS.

APDS

Armour Piercing Discarding Sabot (APDS) is a subcalibre round, meaning its diameter is less than that of the barrel it is being fired from. This move to sub-calibre rounds was driven by a few factors.

As penetration is a function of kinetic energy, the best way to improve penetration is to increase the mass of the projectile and/or the velocity is travels at. Increasing mass was achieved by moving from steel alloys to tungsten alloys, which are around 2.5x denser. This however means a comparable sized AP/APC round would be so heavy as to be unable to be fired at all from most contemporary guns of the time - the rounds had to be made smaller.

As these new rounds were very narrow, they had a lesser cross section creating less drag and so can be fired faster for the same (or marginally greater) mass and, having a narrower profile, exert more penetration on impact - the same force on a smaller surface area has a greater penetrating effect.

To illustrate this, the new 17 pounder APDS projectile weighed 3.5 kg, had a velocity of 1,200 m/s and could penetrate 233 mm of armour at 1,000 m - 50% of the weight of an APCBC, 30% more velocity and 55% greater penetration.

Because APDS rounds are narrower than the barrel of the tank, they need help to be fired without rattling around and breaking the gun and the round itself. The round is held in place by a sabot, which consists of several 'petals' that connect together around the projectile to hold it in place and minimise any yaw within its travel down the barrel, as well as to seal the barrel to ensure accelerating forces are transmitted as fully as possible to the projectile itself.

An APFSDS round's sabot petals discarding shortly after leaving the barrel.

Once it leaves the barrel the petals are no longer needed, as they are heavy and large and so are entirely detrimental to the projectiles velocity, they are engineered to pivot away from the rod itself (hence the term; discarding sabot) and leave the round to travel to the target alone.

So why smoothbore?

Progress, or rather an obstacle to it. As time goes by, continually higher levels of penetration are relentlessly sought. For AP rounds, this ultimately means increasing the length to diameter (L/D) ratio - the longer a projectile is relative to its diameter, the greater the penetration. You also want to make it denser, so that it has even more mass acting on its small diameter cross section when it impacts.

However, the longer and thinner a rod becomes, the more aerodynamically unstable it becomes and the risk of tumbling in flight increases, as well as a general inability to maintain accuracy.

Though spinning a round via rifling is a means to stabilise a projectile, this only works to a certain point, and once the projectile becomes more than circa seven times its diameter, the gyroscopic stabilisation becomes much less effective.

Additionally, the rifling engaging with the round and imparting a spin eats up a portion of the energy of the projectile, parasitically reducing velocity, and the spinning movement through the air also results in faster degradation of velocity than an un-spun projectile of the same dimensions.

To maintain stability but extend the length of the projectile an alternative to gyroscopic rotation is needed, and that alternative is to add fins, creating the Armour Piercing Fin Stabilised Discarding Sabot (APFSDS).

APFSDS

Much like an arrow from a bow or a dart, an APFSDS stabilises itself in flight with 4-6 tail-mounted fins, which keep the round stable throughout its flight without any concerns about maintaining a high rate of rotation.

The fins are not without cost. The fins increase drag to around 150% to 225% of the value were the projectile just a plain rod. Combined with the increasing size and weight of the projectile, gun designers concurrently push for larger and larger charges to maintain velocity (the current velocity norm of c.1,700 - 1,800 m/s

There actually is a deliberate gentle rotation (called the equilibrium spin or equilibrium roll rate) typically of 20-30 revolutions per second, achieved by subtly canting the leading edges of the fins to impart that rate at the anticipated firing velocities. This is to keep the projectile moving consistently through the air in spite of any manufacturing inconsistency - even with the very high tolerances used, a subtle variance in rod or fin geometry can exist and would result in the round veering off course if it were truly static in its rotation. By gently spinning it, it keeps any such asymmetry centred on a rotating axis, diminishing if not completely nullifying it.

There is also a small factor of aeroelasticity, which is that the enormous forces imparted on the rod are such that they can bend the rod slightly into a subtle bow-shape, which diminishes as the rod progresses to its target. This is less evident in denser and stiffer rods, but nonetheless exists and is also mitigated by the equilibrium spin.

Moving to fin stabilisation unlocks essentially unlimited length projectiles (within bounds of reason for a tank gun), and enable the super long APFSDS projectiles we have today, which have extended from approx. 457 mm for a DM13 in 1979 to 745 mm for a DM63 today and 900 mm or more in the latest design being developed.

The latest APFSDS manage the size of these huge penetrators by housing them predominantly within the case of the round, surrounded by the propellant.

A sectioned US M829A4 APFSDS-T round, showing how the penetrator itself occupies the full length of the case to avoid becoming entirely unwieldy in the confines of the fighting compartment and storage racks.

These longer APFSDS rounds are still possible to be fired from rifled guns, for example the British Challenger 2 can fire APFSDS (or "fin" as they are referred to) by fitting synthetic slipping driving bands around the sabot that engage with the rifling as the round travels down the barrel, but slip against the surface of the sabot so that it does not rotate with them.

A British L27A1 CHARM 3 (CHallenger ARMament 3) APFSDS round. Note that british ammunition is 2-part, so the charge is loaded separate from this projectile.

This allows the round to stay largely static (barring the intentional equilibrium spin). The bands snap under the force of the petals trying to separate on leaving the barrel, and then work as any other APFSDS round. This is not ideal, though, as there are a lot more frictional losses with all the materials sliding on one another than if the round just moved down a smooth barrel, which is also reflected in lower barrel lives for rifled than smoothbore guns.

HEAT

There is another reason for moving to smoothbore, which is HEAT (High Explosive Anti-Tank) rounds. These operate 'chemically', meaning their penetration is the result of a reaction, not simple kinetic force.

Specifically, they use a shaped charge to collapse a metal liner (typically copper) inside the warhead into an extraordinarily high velocity (>8,500 m/s, vs c.1,700 m/s for an APFSDS) jet of material that will punch through armour. Note that whilst the shaped charge jet is formed by an explosive action, the resultant penetrator is simply a copper material and has no explosive effect as it penetrates the target.

HEAT rounds are by no means new and predate modern smoothbore tank guns. having been invented before the First World War. They took off as a powerful technology during the war mainly as infantry fired weapons like the German Panzerschreck and Panzerfaust or British PIAT, with a few dabbles in HEAT rounds for tanks like the German 7.5 cm Gr.38 Hl/A fired by the KwK.37 L/24 of the Panzer IV and StuG III, however these were not as effective due to the small calibre of tank guns at the time resulting in quite narrow cones that limited penetration, something that is far less the case with modern 120 mm or greater calibre guns.

The proper formation of that jet is critical, and governed by a few physical rules including the design of the cone and explosive, the distance from the target that the round is initiated, and the stability of the environment the jet is formed in, and progresses through.

The latter is the issue that has in part contributed to the migration to smoothbore guns.

Reasonably intuitively, jets do not form well when they are being spun at very high rpm - the collapsing cone is trying to create a tightly focused jet, but the centrifugal force of the spin is trying to force the jet to disperse.

HEAT-type rounds that are fired from rifled guns have to make a range of efforts to mitigate the spin, one option is to add 'flutes' to the liner that form an opposing spin action, resulting in the opposing spins cancelling each other out and producing a relatively stable jet. This technique is still used in 40 mm HEDP grenades and M789 HEDP rounds fired by the Apache's 30 mm M230.

A fluted shape charge liner.

However, this is all still very suboptimal. The best situation is to have the jet form in a stable environment, and that again is possible with a smoothbore fin stabilised projectile where it is largely static in terms of rotation and can form a clean jet as a result.

What about other round types?

The above missed out a lot of other round types, including SAP, APHE, SAPHE, APCR, HVAP, APCNR, APSV, HE, HESH, AB, ABPD/PD, Cannister, Flechette, Smoke, ATGM and many more (plus practice/TP and -T tracer variants). That's because they aren't really part of the narrative for why smoothbore become the standard for tank guns globally (that last word is aimed at you, HESH lovers).

But, we'll cover all of these in a future primer on medium and large calibre ammunition.

Its also worth acknowledging that rifled guns are by no means dead. As mentioned, Challenger 2 (though not the incoming Challenger 3) use rifled guns, and in lower calibres rifled guns remain a viable choice. 105 mm for example remains a capable calibre and the vaunted L7 rifled gun remains a best-in-class for that calibre. More on that in a future post, too.