"Underslung launcher mechanism designed to deliver an electrically charged shotgun slug; contact with enemy personnel results in temporary, but complete, paralysis."
W-Y 21S Arc Rounds description (from Aliens: Colonial Marines)

The Weyland-Yutani 21S Arc Round is a electrically charged ammunition for use with a Weyland-Yutani underbarrel projectile launcher.


The 21S Arc Round is a 15mm electrically charged cartridge that upon impact with an organic tissue will deliver a high voltage charge that will temporary stuns the target making vulnerable to capture or attack.


  • The 21S Arc Round launcher in Aliens: Colonial Marines is visually identical to the U1 Grenade Launcher unit fitted to Pulse Rifles as standard. Given that the Colonial Marines Technical Manual states the U1 is capable of using a variety of ammunition types, it seems possible the name "H34 Fragmentation Round" launcher is simply a gameplay designation, and that the weapon is in fact a U1 loaded with a different ammo type. The fact the 21S is named after ammunition it fires rather than having a specific designation (unlike the U1) further supports this theory.
    • However this theory is kinda subverted by the caliber difference between the two. This grenade launcher fire 15mm ammunitions while the U1 Grenade Launcher fires 30×71mm projectiles.
  • The 21S is unlocked at Marine Level 5 alongside the MK221 Tactical Shotgun.
  • Although the NSG-23 and the MK221 having the same underbarrel attachment, the NSG-23's one can't fire 21S Arc Round.