Sub-caliber training

Sub-caliber training is used to save wear and expense when training with a larger gun by use of smaller weapons. 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 M120 mortar that allows the mortar to use ammunition or the M49A1 sub-caliber device, which used 7.62×51mm NATO rifle rounds in the M67 recoilless rifle. These devices/weapons have been used for guns as large as the main guns of battleships.