Bio-mass, a radioactive orange-red substance, serves as the ammunition for most of the Drudge weapons, such as the Strike Rifle, Striker, Shrieker, and Warp Pistol. Smaller bio-masses are used as Radiation grenades. Not much else is known about it, except for what Prometheus told Michael Ford: "Drudge weapons utilize a unique bio-mass that can be converted into a variety of damage effects. It's far more versatile than a plasma core and several times as deadly." You can see the bio-mass ball come out of a Drudge "ammo cache" when you activate it.
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