Sub-caliber training is used to save wear and expense when training with a larger gun by use of smaller weapons with identical 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 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 or the M49A1 sub-caliber device, which used 7.62x51mm NATO rifle rounds in the 90 mm (3.5 in) M67 recoilless rifle

M67 recoilless rifle

The M67 recoilless rifle was a 90-mm antitank recoilless rifle made in the United States and later in the Republic of Korea. It could also be employed in an antipersonnel role with the use of the M590 antipersonnel round...

. These devices/weapons have been used for guns as large as the 12–16 in (30.5–40.6 ) main guns of battleship

Battleship

A battleship is a large armored warship with a main battery consisting of heavy caliber guns. Battleships were larger, better armed and armored than cruisers and destroyers. As the largest armed ships in a fleet, battleships were used to attain command of the sea and represented the apex of a...

s.