Modern tank barrels are smoothbore because the most effective anti-armor projectiles perform better without spin. A rifled barrel imparts rotation to stabilize a round in flight, which works well for traditional shell shapes, but the long, dart-like penetrators used by today’s tanks actually lose effectiveness when spun. Smoothbore guns also handle higher velocities, last longer, and can fire a wider variety of ammunition, making them the universal standard for every major military except one holdout that is now switching over.
Spin Hurts Modern Anti-Armor Rounds
The primary ammunition modern tanks use against other tanks is a kinetic energy penetrator: a long, narrow tungsten or depleted uranium dart fired at extremely high speed. These rounds, known as armor-piercing fin-stabilized discarding sabot (APFSDS) projectiles, rely on sheer velocity and density to punch through armor. They use small fins at the rear to stay stable in flight rather than relying on spin.
Spin stabilization works by rotating a projectile around its long axis, like a football spiral. This is effective for compact, roughly symmetrical shapes. But for a long, thin rod, spin creates problems. The rod wants to wobble outward as it spins, and the longer the rod relative to its diameter, the worse this effect becomes. Fin-stabilized penetrators can be made much longer than spin-stabilized ones, and longer penetrators punch through more armor at greater range. U.S. military testing confirmed that APFSDS projectiles penetrate more armor at greater distances than their spin-stabilized predecessors, specifically because fin stabilization allows longer penetrator designs that fly with accuracy and stability.
There’s also a direct performance penalty when these dart-like rounds hit armor while spinning. A spinning penetrator disperses some of its energy laterally on impact rather than driving it all straight through the plate. Removing spin concentrates the full force of impact into the smallest possible cross-section, maximizing penetration depth.
Higher Muzzle Velocities With Less Wear
Smoothbore barrels can fire projectiles at higher velocities partly because there’s less friction. In a rifled barrel, the propellant gases must also do the work of engraving the round into the rifling grooves and spinning it, which absorbs energy. A smoothbore barrel eliminates that friction, letting more of the propellant’s energy translate directly into forward speed. For a kinetic energy round that depends entirely on velocity for its lethality, this matters.
The wear advantage is substantial. Rifling consists of precisely machined grooves that must stay within tight tolerances to remain accurate. Every round fired erodes those grooves, and the worst damage typically occurs right at the origin of rifling where the projectile first engages the lands. Excessive stress between the rifling and the projectile’s driving band accelerates this wear. A smoothbore barrel, lacking these features, degrades more gradually. Estimates put smoothbore barrel life at roughly 1,500 equivalent full charges compared to around 400 for a rifled barrel of similar caliber. That’s nearly four times the service life, which translates directly into lower maintenance costs and less downtime in the field.
Ammunition Flexibility
A smoothbore gun can fire a broader family of ammunition types. Beyond kinetic energy darts, tanks carry high-explosive rounds for use against infantry, buildings, and light vehicles. Some smoothbore guns can also launch anti-tank guided missiles directly through the barrel, like the Ukrainian Kombat missile, which fires from a 125mm smoothbore gun and reaches targets out to 5,000 meters. A rifled barrel would impart spin to a missile that doesn’t want it, complicating guidance and requiring design workarounds.
This versatility means a single gun system can handle engagements against enemy tanks, fortified positions, and lighter vehicles without needing specialized launchers. The ammunition does the adapting, not the gun.
Why the British Held Out (and Stopped)
The most notable exception to the smoothbore consensus was the British Challenger 2, which kept a rifled 120mm gun well into the 2020s. The reasons were more practical than philosophical. Britain had invested heavily in a type of explosive round called HESH (high-explosive squash head), which flattens against a surface before detonating and sends a shockwave through the material. HESH proved particularly effective against concrete structures and was valued for engagements against anything that wasn’t another tank. These rounds need spin to fly accurately, so they need a rifled barrel.
Logistics played an equally large role. The Challenger 2’s gun was designed for backward compatibility with the earlier Challenger 1, and maintaining one family of 120mm ammunition was cheaper than building a parallel supply chain. The decision to keep rifled guns wasn’t driven by any performance advantage over smoothbore designs. It was driven by existing stockpiles and a desire to use domestic components.
That era is ending. The Challenger 3, Britain’s next main battle tank, replaces the rifled gun with a 120mm L55A1 smoothbore, the same gun family used by Germany’s Leopard 2. The switch gives Britain ammunition commonality with other NATO members, meaning allied nations can share stocks in a conflict. A new kinetic energy round is being co-developed with Rheinmetall for both the Challenger 3 and the Leopard 2, cementing the smoothbore as the last remaining standard for Western tank guns.
Why Rifling Still Exists Elsewhere
Rifled barrels haven’t disappeared from military use. They remain standard on smaller caliber weapons, from infantry rifles to autocannons on armored fighting vehicles. At those calibers and velocities, conventional bullets and shells benefit from spin stabilization. The physics that make spin counterproductive for a long-rod penetrator fired at 1,700 meters per second don’t apply to a compact rifle bullet at 900 meters per second. Rifling also remains on most artillery pieces, where shell shapes are traditional and accuracy at long range depends on stable rotation.
The shift to smoothbore is specific to tank main guns because tanks fire the kind of ammunition, at the kind of velocities, where rifling becomes a liability rather than an asset. Every major tank designed or upgraded since the 1980s reflects that reality.