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).

To assure compliance with safety requirements in newer weapon systems, which are capable of using the M203 Propelling Charge, a conversion to TNT fill in lieu of Composition B was introduced in September 1977 with Type Classification of the M549A1.

[citation needed] The 155 mm M549/M549A1 HERA is a separate-loading munition with a two-part forged shell body.

Before loading, the lifting plug is replaced by a fuse, and the protector cap over the rocket-motor nozzle is removed; the 155 mm M549/M549A1 HERA is not intended to be fired in the 'rocket-off' mode.

At the instant of firing, the propellant gases ignite a pyrotechnic delay train in the rocket motor.

[citation needed] When the weapon is fired, the rotating band engages the barrel rifling to impart spin to the projectile for stability in flight.

The obturator and rotating band form a seal to prevent leakage of gas pressure past the projectile.

The XM1113 increases range from 30 to 40 km (19 to 25 mi) from a 39-caliber barrel using a large high-performance rocket motor delivering nearly three times more thrust, along with a streamlined exterior profile shape for lower drag.