The M549 is a High-Explosive Rocket Assisted (HERA) 155 mm howitzer round developed for use by the US Military in order to add additional range to standard howitzers, with a maximum range 30.1 km from a M198 howitzer. The projectile has two distinctive pre-assembled components—the high explosive warhead and the rocket motor, making it a form of rocket-assisted projectile. The warhead is fabricated from high fragmentation steel for increased effectiveness in terms of damage caused to target and contains a bulk-filled explosive (either TNT or Composition B).