Sorry it doesn’t exactly work as you’re expecting it to. I will attempt to explain it, but again it doesn’t follow the traditional mechanics of most FPS games. Besides, knowing how much ammo it takes to kill isn’t that effective, the much more useful thing to know is a weapon’s Time to Kill
There are two things a player has, Shield and Health.
A DMR takes 4 shots to drop somebody’s shields. Because the DMR also registers headshots (Counter-intuitive, NOT ALL weapons do this), once a target’s shields are dropped, if you shoot them ONCE in the head with a head-shot capable weapon, they instantly die. If a target has lost their shields and you continue to shoot them with the DMR, but aim for their body…you drop their health until it depletes and they die.
So in total, a DMR takes 5 shots to kill somebody if you finish them with a headshot. It should also be noted that when performing the first 4 shots, it is the same damage regardless of whether or not you shoot them in the body, or the head.
Some games deal different amounts of damage depending on whether the shot hits a limb, torso, head. Halo only cares about headshots vs nonheadshots.
The following are capable of inflicting instant kills…
Rockets
Shotgun
Sword
Hammer
Sniper Rifle (Headshot only, if body shield does 1 shield worth of damage)
Spartan Laser
Plasma Grenades (If you stick them, or it is close enough)
Plasma Launcher grenades
Grenade Launcher
Meleeing somebody in the back
Meleeing somebody while you are holding a bomb, oddball, or the flag
Guass turret
Scorpion Tank main gun
Wraith mortar
Banshee bomb
Splattering with any vehicle, if fast enough, not sure if scorpions can splatter.
The following are the combo attacks, meaning a 2 step attack which performed correctly results in instant death. Basically these combo attacks consist of using something to drop an opponent’s shields, then killing them with a finisher, which is either a Melee or a Headshot (With a weapon which registers headshots, will be listed below)
Plasma Pistol overcharge burst, finisher. It should be noted that the overcharge doesn’t affect health, but the individual shots do
Sniper shot to the body, finisher
Melee, finisher
Grenade, finisher
Shotgun, finisher. The shotgun’s instant kill range is a bit shorter than Halo 3’s. You can fire a shot then virtually immediately melee for a near instant kill
Armor Lock release, finisher (Typically melee)
EMP effect from the Grenade Launcher, melee
The following weapons register headshots:
DMR
Needle Rifle
Pistol
Sniper Rifle
So the most effective methods of killing opposing players are:
Instant Kill (If you’re in the appropriate range)
Combo Kill
Team Shot (Typically with the DMR)