Tanks and their big guns get all the attention, but Russia's smaller cannons are wreaking havoc against Ukrainian troops

• International attention has been drawn to tank warfare in Ukraine.
• Tanks have big guns, but medium-caliber cannons on other armored vehicles can be just as deadly.
• Those smaller cannons have "shaped combat" for both Russian and Ukrainian forces, one expert says.

While tanks and their big guns have been getting the lion's share of attention in Ukraine, another weapon is proving equally deadly.

Medium-caliber cannon — the 20-mm to 40-mm cannon found on infantry fighting vehicles, air-defense vehicles, and other platforms around the world — have been wreaking damage against infantry and other targets.

In Ukraine, both sides are using Soviet-era 2A42 30-mm autocannon that are outfitted on the Russian-designed vehicles used by both armies, including the BMP-2, BMP-3 and BMD-4M troop carriers, as well as Ukrainian versions of the BTR-3 and BTR-4, land-warfare expert Sam Cranny-Evans wrote in European Security and Defence in January.

Those cannons are in widespread use, have "ferocious" firepower, and they have "shaped combat" for both Russian and Ukrainian forces, wrote Cranny-Evans, who previously was a research fellow at the Royal United Services Institute, a British think tank.

Cranny-Evans cited a Ukrainian soldier who complained last year that Russian armored personnel carriers "are worse than tanks. The gun on them fires faster and they carry soldiers."

"Wounds from these are horrible. A whole leg can be removed. A shot to the body makes someone almost explode," the Ukrainian soldier said, adding that Russian APCs "are easy to destroy but not good to fight directly."

Medium-caliber cannon can engage infantry from a distance or even damage a tank — both of which are uses observed in Ukraine, "and it is apparent that infantry formations fare poorly against cannons if they are not supported," Cranny-Evans wrote. "They also play an important role in urban warfare as they are capable of suppressing positions from ranges that are likely to be beyond the reach of the standard shoulder-fired anti-armor weapons carried by infantry formations."

People instinctively view the biggest cannon as the best in terms of firepower, but it's not that straightforward. A main battle tank like the US-made M-1 or the Soviet-designed T-72 are armed with 120-mm or 125-mm guns that can destroy other tanks from miles away. But those tanks carry a limited number of cannon rounds and their main guns have a limited rate of fire.

Moreover, their main guns are ill-suited for many targets on the battlefield. During World War II, 80% of targets engaged by the M4 Sherman tank were soft targets such as infantry, anti-tank guns, and bunkers. To send tanks after dispersed targets like infantry is like hunting rabbits with a howitzer.

That's why tanks have machine guns in the 12.7-mm and 7.62-mm range, to engage personnel or to hit soft vehicles, such as trucks, that don't require a big cannon round.

Medium-caliber, rapid-fire cannon, such as the M242 Bushmaster 25-mm chain gun on the US's M2 Bradley infantry fighting vehicle, combine a rate of fire like that of a machine gun with the heavier shells used by a cannon. The 2A42 can pump out up 500 rounds a minute, while the 27A2 model can shoot 330 rounds.

These weapons can also fire a variety of ammunition, including armor-piercing and high-explosive rounds. The M242 Bushmaster can even fire depleted-uranium shells against tanks. Bradleys famously used their 25-mm guns to suppress Iraqi T-72 tanks, buying them time to destroy the Iraqi tanks with the M2's TOW anti-tank missiles.

This doesn't mean that infantry fighting vehicles, if they have a choice, should pick a fight with heavily armored tanks, but medium cannon have proven their value. Britain found 30-mm cannon useful in the Falklands, Cranny-Evans wrote, adding that these weapons "were also used as a form of precision direct fire in Afghanistan where they served to quickly suppress Taliban forces, and similar outcomes have been reported by French forces fighting in Mali."

It's now common for infantry fighting and reconnaissance vehicles to carry guns that would have been the main armament of tanks in the early years of World War II. Future vehicles will still probably be armed with medium cannon, though with some innovations.

The US and Russia are looking at heavier 50-mm or 57-mm guns that use special ammunition such as the Supershot, while cased telescoped cannon offer the prospect of smaller and lighter weapons. The US Army also plans to arm the Stryker with a 30-mm gun, while France and Britain have ordered BAE's 40-mm Cased Telescoped Cannon for their light armored vehicles.

However, NATO, Russia, and China (which copies Russian weapons) are largely using medium-caliber cannon designed in the 1970s. As Russia is still using Cold War-era armored vehicles vulnerable to NATO's current medium cannon — among those Soviet-designed vehicles' many flaws — there may not be much impetus among Western militaries to develop new weapons.

Nonetheless, Ukraine has become a laboratory to demonstrate what happens when modern mechanized armies meet in battle. The effectiveness of medium-caliber cannons suggest that as armored vehicles evolve, those guns will remain part of their armament.

Michael Peck is a defense writer whose work has appeared in Forbes, Defense News, Foreign Policy magazine, and other publications. He holds a master's in political science. Follow him on Twitter and LinkedIn.