Ammo Gauge

The Ammo Gauge is a feature of the Heads-Up Display exclusive to Metroid Prime 2: Echoes. It tracks the amount of Beam Ammo that Samus is carrying, represented by two bars of squares, one purple and one white. The purple bar represents Dark Beam ammo, and the white bar represents Light Beam ammo. The gauge depletes as ammo is consumed, forcing Samus to conserve her ammunition and seek out refill pickups, Ammo Stations or return to her Gunship to restore her reserves. Initially, the maximum Samus can carry for each Beam is fifty, but Beam Ammo Expansions can be acquired that will expand her reserves to carry up to 250 units of ammunition. As with the Missile Counter, the Ammo Gauge will warn Samus when her ammo is low, and then when it is depleted. The respective Beam(s) can still be charged to fire a normal shot, as a weak form of defense or to open a door or energize a Portal of the appropriate polarity.

Metroid Prime 2: Echoes
"Once Samus acquires the Light and Dark Beams, these gauges will track how much ammo she has for each weapon."

Metroid Prime Trilogy
"Purple line on the left shows the amount of Dark Beam ammo remaining, and white line shows the amount of Light Beam ammo remaining. The numbers show the exact amount of each ammo remaining."