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Aim directly at the ground with your Battle Rifle and walk forward. The projectiles will hit your foot and be considered a suicide (similar to sticking one's own foot with a plasma grenade).
It is also possible for one to shoot themself in the head. To do this, run and shoot twice straight ahead,then switch weapons. If performed quickly, the ammo will hit the firer in the head for an instant death (this is a variation of the Guardian Glitch).
You can also commit suicide with a Sniper Rifle. Go to the front of a Scorpion and shoot at it in such a way where it will ricochet off the body and off the cannon. You will see the bullet hit the body, hit the cannon then hit you. Depending on the angle you do it in, you can either get a body shot, or a headshot. Either one is a suicide.