The US Army is considering various systems to better shield tanks and armored vehicles from RPGs, antitank missiles, and other enemy fire.

But the latest version of the RPG, a staple in the arsenals of Russia and other forces, may already be a step ahead of the active-protection systems the US may soon adopt.

The Pentagon has purchased active-protection systems to test out on Abrams tanks and Bradley and Stryker armored vehicles, and may even mount them on lighter vehicles, like the successor to the Humvee, according to a report from Scout Warrior.

“The Army is looking at a range of domestically produced and allied international solutions from companies participating in the Army’s Modular Active Protection Systems (MAPS) program,” an Army official told Scout Warrior.

The Army intends to outfit Abrams tanks with the Israeli-made Trophy APS and Bradley vehicles with the Iron Fist system, which is also Israeli-made. It plans to put the US-made Iron Curtain system on Stryker vehicles. (The Army leased several of the Trophy systems last spring, working with the Marine Corps to test them.)

“The one that is farthest along in terms of installing it is … Trophy on Abrams,” Lt. Gen. John Murray, the Army’s deputy chief of staff, said in a statement. “We’re getting some pretty … good results. It adds to the protection level of the tank.”

The US’s look to APS comes as other countries adopt the technology.

Israeli’s Merkava comes standard with the Trophy, as does Russia’s new T-14 Armata. Both Israel’s and Russia’s tanks, as well as the UK’s Challenger 2, are considered by US officials to be close to or at parity with the US’s mainstay, the Abrams tank. (Though some officials don’t consider the Armata fielded.)

As militaries have adopted active-protection systems and other means to up-armor tanks, arms makers have looked for new antitank weaponry to counter them. Whenever US vehicles equipped with APS join similarly outfitted vehicles in the field, they will face a new challenge from an old foe, the RPG.

Russian arms manufacturers first introduced the RPG — short for Ruchnoy Protivotankovyy Granatomet, meaning “handheld antitank grenade launcher,” not “rocket-propelled grenade” — in 1949, updating it over the decades since.

The most recent variant, the RPG-30, unveiled in 2008, has a 105 mm tandem high explosive antitank round, and features a second, smaller-caliber projectile meant to bait the active-protection systems that have become common on armored vehicles in recent years.

A tandem HEAT round carries two explosive charges. One neutralizes a vehicle’s reactive armor (which uses explosions to counter incoming projectiles), and the other is designed to penetrate the armor of the vehicle itself.

“The novelty of the Russian rocket launcher is that two rockets are fired at the target at the same time. One is a so-called ‘agent provocateur’ 42 mm in caliber, followed a bit later by a primary 105-mm tandem warhead rocket,” Vladimir Porkhachyov, the director general of arms manufacturer NPO Bazalt, told Russian state news agency Tass of the RPG-30 in September 2015.

The RPG-30 reportedly cleared testing and went into active service with the Russian military sometime between 2012 and 2013. At that point, according to a 2015 report by Russian state-owned outlet Sputnik, the Pentagon put it on its list of “asymmetrical threats to the US armed forces.”

The effectiveness of the RPG-30 against active-protection systems, and whether those systems need be upgraded to adapt to the RPG-30 and similar munitions, remains to be seen. But the RPG — though limited by the size of its warhead — has long been potent on the battlefield, even against modern tanks.

The previous model, the RPG-29, was introduced in 1991 and is still in service with the Russian armed forces. It fires a 105 mm tandem HEAT round and can also fire a thermobaric fuel-air round against bunkers and buildings.

Russian RPG-29s were used by Hezbollah in the mid-2000s, deployed against Israeli tanks and personnel during the 2006 Lebanon War.

According to a Haaretz reportfrom the time, Hezbollah antitank teams using RPG-29s managed on some occasions to get through the armor of Israel’s advanced Merkava tanks.

In other cases, Hezbollah fighters used the RPG-29 to fire on buildings containing Israeli troops, penetrating the walls.

“The majority of Israel Defense Forces ground troops casualties, both infantry and armored, were the result of special antitank units of Hezbollah,” which used other antitank missiles as well, according to the Haaretz report, published in the final days of the conflict and citing intelligence sources.

Those RPG-29s were reportedly supplied to Hezbollah by the Syrian military, which got them from Russia. Moscow disputed those origins, however, with some suggesting they were exported from former Communist bloc countries after the fall of the Soviet Union.

In August 2006, a RPG-29 was used successfully against a British Challenger 2 tank in southern Iraq.

During operations in Al Amarah, an RPG-29 rocket defeated the reactive armor installed on the Challenger, penetrating the driver’s cabin and blowing off half of one soldier’s foot and wounding several other troops.

UK military officials were accused of a cover-up in 2007, after it emerged that they hadn’t reported the August 2006 incident.

Two years later, during fighting in Baghdad’s Sadr City — a Shiite neighborhood in the Iraqi capital — a US M1 Abrams tank was damaged by an RPG-29. (The US has long avoided reactive armor systems but accepted them in recent years as a cheap, easy way to up-armor vulnerable parts of the Abrams, particularly against RPGs.)

During fighting in Iraq, RPG-29s penetrated the armor on the Abrams tanks twice and the Challenger once, according to The National Interest. Other Abrams tanks in Iraq were knocked out by antitank missiles, like the Russian-made AT-14 Kornet.

The threat goes beyond tanks. Seven of eight US Army helicopters shot down in Afghanistan between 2001 and 2009 were brought down by RPGs.

RPGs remain in service around the world, filling the arsenals of both state and non-state actors, according to the Small Arms Survey. The weapon and parts for it have popped in arms bazaars in Libya in recent years.

The RPG-7, the RPG-29’s predecessor, would be or would likely be used by forces in Eastern Europe, the Middle East and North Africa, sub-Saharan Africa, as well as Central, South, and East Asia.

Regular and irregular forces in Latin America also have RPGs, and the weapons have made their way into the hands of criminal groups in the region. The Jalisco New Generation cartel reportedly used one to down a Mexican military helicopter in early 2015.