Sub-caliber ammunition

It can for example allow for much higher muzzle velocities due to smaller lighter projectiles being fired from relatively larger propellant charges,[1] but it can also lower the cost of ammunition due to less material being used to produce the round compared to a full-caliber ammunition etc.

[1] The most traditional way to fire sub-caliber ammunition is to fit the projectile with an expendable sabot.

[1] A common method during World War Two, often called the Gerlich-, Littlejohn- or tapered bore principle, was to fit sub-caliber ammunition with soft metal flanges filling out the missing caliber and then fire them from squeeze bore barrels.

When fired the flanges on the projectile would fold inwards as it travels through the reducing inner diameter of the squeeze bore.

This is called sub-caliber training and it is used to lower the cost of training with large caliber weapons by allowing them to fire cheaper lower caliber ammunition and to not put wear on the original barrel.

Sub-caliber armour-piercing discarding sabot projectile. Here seen with and without its sabot as well as its internal tungsten core.
Sub-caliber squeeze bore projectiles. Here seen in two examples: an armor-piercing composite non-rigid projectile with a tungsten core, and a regular armor-piercing non-rigid projectile without a tungsten core.