155 mm (6.1″) is a common and NATO-standard artillery calibre.
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Land warfare
Since the late 20th century, most NATO armies have centred on 155 mm weapons as having a good compromise between range and destructive power whilst having a single calibre simplifies logistics; leading to the obsolescence of calibres such as 175 mm and 203 mm – although some forces retain 105 mm guns for their portability. Russian/Soviet guns and those of other aligned countries tend to use 152 mm weapons in similar roles.
Naval use
155mm has not found widespread use among naval forces despite its ubiquity on land, with most NATO and aligned navies using 76 mm (3"), 100 mm (3.9"), 113 mm (4.5"), or 127 mm (5") 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 gun to give increased firepower and a common calibre between the Royal Navy and the British Army. However, despite superficially appearing to be superior due to a comparison of round diameters, when firing conventional ammunition the smaller 4.5" naval gun is comparable to the standard 155 mm gun-howitzer of the British Army. The standard shell from a 4.5" naval gun has the same, if not better, range and carries twice the HE payload of a standard 155 mm round; only by using rocket-assisted projectiles (RAP) can most 155 mm guns better the range of the 4.5", doing so by sacrificing 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 calibre (for example the Mark 8 has a barrel length of 55 calibres, while the standard AS-90 self propelled gun has a barrel length of 39 calibres). 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. In addition, even without active cooling, the heavier naval gun barrels allow for a superior sustained rate of fire compared to field guns, and this is exploited with an autoloading system with a capacity of several hundred rounds. The 155 mm is superior to the 4.5" in relation to cannon-launched guided projectiles (CLGP); because the 155 mm round is fired at a lower velocity it is much easier for their internal electronic guidance systems to survive being fired.
The US Navy's Advanced Gun System also uses a 155 mm calibre, although it is not compatible with NATO-standard 155 mm ammunition.