Sub-caliber round

A sub-caliber round is a round which has a smaller diameter compared to the internal barrel diameter from which it is fired. Firing sub-caliber ammunition has several potential benefits compared to full-caliber ammunition. For example it can allow for much higher muzzle velocities due to smaller lighter rounds being fired from relatively larger propellant charges. It can also lower the cost of ammunition due to less material being used to produce the round compared to a full-caliber round etc.
Several methods exists to be able to fire sub-caliber ammunition.
Sabot method.
The most traditional way to fire sub-caliber ammunition is to fit the round with an expendable sabot. The sabot is a device which fills out the missing caliber when the round is fired and then leaves the round once it has left the barrel.
Flange method.
A common method during world war two was to fit sub-caliber rounds with soft metal flanges filling out the missing caliber and then fire them from squeeze bore barrels. Squeeze bore barrels progressively decreases its internal barrel diameter towards the muzzle resulting in a reduced final internal diameter. When fired the flanges on the round would fold inwards as the round travels through the reducing inner barrel diameter.
Sub-caliber barrel.
A third method is to simply fit a smaller barrel into the original full caliber weapon which then allows the firing of smaller caliber cartridges. 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.