155 mm (6.1 in) is a NATO-standard artillery shell caliber that is used in many field guns, howitzers, and gun-howitzers. It is defined in AOP-29 part 1 with reference to STANAG 4425.

This article is missing information about scope of NATO standardization (does it require separate loading?); extended range full bore shell; further Ballistic Memorandums of Understanding among some NATO members (L/39, L/52, etc.). (November 2021)

The 155 mm (6.1 in) caliber originated in France after the Franco-Prussian War (1870–1871). A French artillery committee met on 2 February 1874 to discuss new models for French fortress and siege artillery, among which there was a weapon in the 14–16 cm (5.5–6.3 in) caliber range (later it became known as the De Bange 155 mm cannon). After several meetings, on 16 April 1874 the committee settled on the 155 mm (6.1 in) caliber (in the subsequent program-letter of the committee, dating from 21 April 1874, the caliber was for the first time expressed as 155 mm (6.1 in)).

Since the early 21st century, most NATO armies have adopted 155 mm (6.1 in) weapons as an all-purpose standard. They are seen as striking a good compromise between range and power, while using just a single caliber greatly simplifies the logistics burden. This has led to the obsolescence of larger caliber artillery shells such as the 175 mm (6.9 in) and 203 mm (8.0 in). Some militaries continue to retain the smaller 105 mm (4.1 in) weapons for their light weight and greater portability. Russia and former Eastern Bloc countries tend to use 122 mm (4.8 in), 130 mm (5.1 in), and 152 mm (6.0 in) artillery in similar roles.

Since the end of World War II, the 155 mm (6.1 in) caliber has not found any use among naval forces despite its ubiquity on land with most NATO and aligned navies using 76 mm (3.0 in), 100 mm (3.9 in), 114 mm (4.5 in), or 127 mm (5.0 in) guns on modern warships. At one point the British Ministry of Defence studied "up-gunning" the Royal Navy's 4.5 inch Mark 8 naval guns to give increased firepower and a common caliber between the Royal Navy and the British Army. Despite superficially appearing to be inferior based on a simple comparison of round diameters, when firing conventional ammunition the smaller, 4.5 inch Mark 8 naval gun is comparable to the standard 155 mm (6.1 in) gun-howitzer of the British Army. The standard shell from a 4.5 inch Mark 8 naval gun has the same, if not better, range. Only by using rocket-assisted projectiles (RAPs) can most 155 mm (6.1 in) guns have comparable range to the 4.5 inch Mark 8 naval gun and by doing so there is a reduction in the payload. This is because naval guns can be built much more strongly than land-based self-propelled gun-howitzers, and have much longer barrels in relation to caliber (for example the 4.5 inch Mark 8 naval gun has a barrel length of 55 calibers, while the standard AS-90 self-propelled gun has a barrel length of 39 calibers). This allows naval guns to fire heavier shells in comparison to shell diameter and to use larger propellant charges in relation to shell weight, leading to greater projectile velocities. Even without active cooling, the heavier naval gun barrels allow a faster sustained rate of fire than field guns, and this is exploited with an autoloading system with a capacity of several hundred rounds. The 155 mm (6.1 in) is better than the 4.5 inch Mark 8 naval gun for firing cannon-launched guided projectiles (CLGP) as the lower velocity of the 155 mm (6.1 in) shell makes it much easier for the projectiles' internal electronic guidance systems to survive being fired.

While the US Navy's Advanced Gun System (AGS) also uses a 155 mm (6.1 in) caliber, it is not compatible with NATO-standard 155 mm (6.1 in) ammunition. Only one type of ammunition was ever developed and procurement was discontinued in 2016 due to its high cost, making the AGS unusable.^[1]

As of February 2023^[update]–March 2023, Ukraine was firing up to 10,000 artillery shells per day,^[37] with the average monthly rate of 90,000–110,000 of 155 mm shells.^[38] In March 2023, the Ukrainian defense minister asked allies for 250,000 of such shells per month.^[39]

Before the start of the large-scale Russian invasion of Ukraine (2022), the United States produced 14,400 shells per month.^[37] As of March 2023^[update], the rate has increased to 20,000 per month.^[38]

Germany's Rheinmetall was producing 60,000–70,000 per year in 2022.^[37]

Ukraine has a domestic production of shells. As of December 2022, the production rate was "in the thousands".^[40]

The US declared its plans to increase the production to 90,000 per month,^[37] to reach 1,000,000 shells per year in 2025.^[41]

Rheinmetall said it was ready to boost production to 500,000 per year.^[37]

In summer 2023, the EU approved a plan that provides for the production of 650,000 large-calibre ammunition per year, and pledged to supply one million artillery shells to Ukraine over the next 12 months.^[41]