Cadet rifle

A cadet rifle is a rifle used by military cadets and others for basic firearms and marksmanship training. Generally .22 caliber and bolt-action, they also come in semi-automatic versions. They are often miniature .22 caliber versions of standard issue service rifles.
Older 19th century cadet rifles were simply smaller and lighter versions of standard issue service rifles designed to fire reduced power cartridges.
Conversion kits.
.22 caliber conversion kits are commonly used on standard M16 rifles and M4 carbines. As .22LR ammunition is cheaper and less powerful than the standard 5.56x45mm ammo, this allows for cheaper target practice on indoor pistol ranges.