Sub-caliber training is used to save wear and expense when training with a larger gun by use of smaller weapons (sometimes, but not always, with very similar ballistic characteristics). The smaller weapons could be inserted into the larger weapon's barrel, externally attached to the barrel or mounted above the weapon.

Examples include 2.25-Inch Sub-Caliber Aircraft Rocket to train aircraft pilots to shoot aerial rockets that emerged during WWII, or the M303 Sub-Caliber insert for the 120 mm (4.7 in) M120 mortar that allows the mortar to use 81 mm (3.2 in) ammunition^[1] or the M49A1 sub-caliber device, which used 7.62×51mm NATO rifle rounds in the 90 mm (3.5 in) M67 recoilless rifle. These devices/weapons have been used for guns as large as the 12–16-inch (304.8–406.4 mm) main guns of battleships.