Cartridge overall length, or "COL", is important to safe functioning of reloads in firearms.
The maximum overall length is dictated by the need to fit into a box magazine of standard manufacture.
However, for competition purposes during off-hand and slow fire prone match stages, the .223 Remington is loaded one cartridge at a time into the rifle's receiver.
These cartridges can be safely loaded to a length that has the ogive portion of the bullet just touching the rifle's lands.
Many competitive shooters will make these cartridges 0.005" less than the truly maximum allowable overall length, for the sake of safety.