The Bio-Rifle is a weapon which fires biological slime to damage opponents.
Primary and Secondary Fire
Primary fire launches small blobs of slime in quick succession, which stick to walls or the floor, and explode after a while. Any player who touches one of these (including having the blobs fired directly at them) takes damage.
Secondary fire builds up a big blob of slime depending on how long you hold the button down for. At its biggest, this blob can be lethal if it hits an enemy directly.
Strategy
The Bio-Rifle doesn't hold that much ammo, nor is it that powerful. Overall, it's quite useless. If you need to use it, the secondary fire is the best to use because you can quite easily kill with one or two large blobs of slime providing you have enough time and good enough aim to hit a foe with them.