Sub-caliber training

{{Notability|date=February 2022}}

File:Iceland, Sanskeid Range. 37mm sub-caliber mounted on a 75mm field howitzer, 19th and 21st Field Artillery Regiment.jpg sub-caliber mounted on a M116 howitzer during range practice in Iceland, 1943]]

File:MLRS firing on the ranges at Otterburn. MOD 45158572.jpg (228 mm) firing 70-mm Reduced Range Practice Rockets L1A2, 2015]]

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 {{convert|120|mm|1|abbr=on}} M120 mortar that allows the mortar to use {{convert|81|mm|1|abbr=on}} ammunition[https://www.marines.mil/Portals/1/Publications/FM%2023-90%20W%20CH%201.pdf MORTARS - DEPARTMENTS OF THE ARMY TECHNICAL ORDER 11W2-5-13-21] or the M49A1 sub-caliber device, which used 7.62×51mm NATO rifle rounds in the {{convert|90|mm|1|abbr=on}} M67 recoilless rifle. These devices/weapons have been used for guns as large as the {{convert|12|-|16|in|1|adj=on|sp=us}} main guns of battleships.