WASHINGTON (AP) — The 155 mm howitzer round is one of the most requested artillery munitions of the war in Ukraine. Already the U.S. has shipped more than 1.5 million rounds to Ukraine, but Kyiv is still seeking more.

A look at why this particular munition is so commonly used, and why it's been so critical to the war in Ukraine.

WHAT IS THE 155 MM?

Essentially, the 155 mm round is a very big bullet, made up of four parts: the detonating fuse, projectile, propellant and primer.

Each round is about 2 feet (60 centimeters) long, weighs about 100 pounds (45 kilograms), and is 155 mm, or 6.1 inches, in diameter. They are used in howitzer systems, which are towed large guns that are identified by the range of the angle of fire that their barrels can be set to.

The 155 mm shells can be configured in many ways: They can be packed with highly explosive material, use precision guided systems, pierce armor or produce high fragmentation.

Past variants have included smoke rounds to obscure troop movement and illumination rounds to expose an enemy’s position.

“The 155 mm round and the similar Soviet-era 152 mm rounds are so popular because they provide a good balance between range and warhead size,” said Ryan Brobst, a research analyst at the Foundation for the Defense of Democracies. “If you have too small a shell, it won’t do enough damage and go as far. If you have a larger shell, you can’t necessarily fire it as far. This is the most common middle ground, and that’s why it’s so widely used.”

155 MM HISTORY

The French first developed the 155 mm round to respond to World War I's extensive trench warfare, and early versions included gas shells, Keri Pleasant, historian for the Army’s Joint Munitions Command, said in a statement to The Associated Press.

As World War I continued, the 155 mm gun became the most common artillery piece used by the Allies, Pleasant said, and the U.S. Army later adopted it as its standard field heavy artillery piece.

The U.S. military fielded its own version, the M1, for World War II. After the war, the new NATO alliance adopted the 155 mm as its artillery standard.

By the Korean War, the round had been modified again, with a cluster munition variant. “The round contained 88 submunitions, which were dispersed over a wide area to destroy vehicles, equipment, and personnel,” Pleasant said.

ITS USE IN UKRAINE

Howitzer fires can strike targets up to 15 to 20 miles (24 to 32 kilometers) away, depending on what type of round and firing system is used, which makes them highly valued by ground forces to take out enemy targets from a protected distance.