This time I organized it in a more coherant way, gave reasons why the information is important, and added some more new stuff. Additional things at the end of each section.
Maximum Range - Human Projectiles
This is important, because you can still attack, even if your reticule hasn't turned red yet. Also, only in the shotgun does the damage decay over longer distances.
M9 HE-DP Frag Grenade: 95 Meters
(Pistol) 12.7mm S-AP, HE Round: 123 Meters
(Assault Rifle) 7.62mm AP Round: 125 Meters
(Shotgun) 3.5", 8-guage Magnum Round: 125 Meters
(Sniper Rifle) 14.5mm AP, FSDS Round: 2.3+ Kilometers
(Rocket Launcher) 102mm SC, HE Rocket: 390 Meters
(LAAG)12.7x99mm AP Round: 310 Meters
Scorpion Tank Main Cannon: N/A
Scorpion Tank Secondary Cannon: 125 Meters
Frag Grenade: Tested on even ground. Although, for every meter higher you are than your target area, you get one extra meter to the maximum range. Stand on a friend's head for two extra meters. Each extra friend you stand on gives you two extra meters (per freind).
Pistol: Yes, 'HE' stands for 'High Explosive'. This bullet does explode on impact.
Assault Rifle: We all know that you should fire short bursts. Just do it. Everything dies slightly faster if you this way. Ignoring this spreads the impacts outside of the reticule, which SHOULD be the size of the shotgun's reticule. Also, full-auto makes your rate of fire erratic. One more thing, if you must go full-auto: The bullets tend to impact more often in the right half of the reticule.
Shotgun: Yes, the buckshot does travel this far, but you'd need to be either stupid or desperate to use a shotgun from this distance...
Sniper Rifle: I was unable to test this, until hipboydan posted the link to the sniper rifle page. The stats of the gun were on that page; thank you hipboydan.
Rocket Launcher: It goes 390 meters, and blows up. It seems that the all-powerful killer has a safety feature; you miss the target (Aim for the feet, goddammit!), and it blows itself up after a while, like a self-destructing mine.
LAAG: Hmmm... the same cartridge length as the pistol(12.7mm)... Intriguing. Also, this time 'AP' stands for 'Armor Penetrating'. Armor peircing bullets peirce their way through with bullet shape, and armor penetrating bullets force their way through with power(cartridge length).
Scorpion Main Cannon: I don't think this gun has a visible range, for a few reasons. Judging by the speed of the projectiles, I believe that it can travel several (and I mean SEVERAL!) kilometers before it hits it's invisible wall. You wouldn't be able to see the target at this distance.
Scorpion Secondary Cannon: Even less accurate than the shotgun. I suppose you have a 80-90° cone of fire. It should save you from any amatuer snipers by knocking them out of zoom mode. More experienced snipers don't hesitate anymore.
Maximum Range - Covenant Weapons
This is here because the Covenant weapons got jealous. They started whining that the Human weapons got a section, and why couldn't they? I had to obey.
Plasma Rifle: 95 Meters
Plasma Pistol, Normal: 155 Meters
Plasma Pistol, Overcharge: 125 Meters
Needler: 61.7 Meters
Plasma Grenade: 95 Meters
Mounted Vehicle Weapon: 155 Meters
Fuel Rod Cannon: 10 to 60 Meters
Wraith Mortar: 10 to 150 Meters
Plasma Rifle: Approximately 200 shots can be fired from a fully charged rifle. This means two shots per unit of charge. You've fired about 185 by the time you reach a 15% charge.
Plasma Pistol: Approximately 500 shots can be fired from a fully charged pistol. This means 5 shots per unit of charge. An overcharge blast drains 11% of the charge each time. This means that you forfeit 55 shots per overcharge. Choose wisely.
Needler: The farthest a needle can travel, and still lodge itself into the target's heart and explode, is 61.7 meters. It can still hurt the target at 62 meters, but that is because the needle explodes on it's invisible wall. It will track the target at a range of 71.5 meters, but it doesn't have a chance of hurting the target unless the target is charging towards you, and gets within 62 meters in time. Also, the Needler is not a plasma weapon. It's probably one of the technologies they stole/adapted.
Plasma Grenade: Tested on even ground. Although, for every meter higher you are than your target area, you get one extra meter to the maximum range. Stand on a friend's head for two extra meters. Each extra friend you stand on gives you two extra meters (per stacked friend).
Vehicle-mounted Weapon: This is the weapon of the Ghost, Banshee, Shade Stationary Gun and the Covie dropship. It just looks different because of the way they arranged the plasma dischargers.
Fuel Rod Cannon: Untested, of course. I've gotten pretty good at eyeballing distances on Halo, and 10 meters appears to be the distance at which a Hunter or Spec. Ops Grunt stops firing at you. The banshee's FRC does not have a range, because you are basically dropping ballistic missiles. I suppose the FR blast lands 35 meters in front of you. Also, the FR blast cannot be dropped from an altitude greater than 170 meters.
Wraith Mortar: Again, untested. It appears that they stop firing at you when you are 10 meters away. I remember once parking my ghost behind an atoll, relatively 180 meters from the area with wraiths... I accidentally let one see me, and it started shooting at me, but the arcs always landed 30 meters away from me.
Distances At Which Your Targeting Reticule Turns Red.
When your reticule turns red, you are within the appropriate range to attack from.
M6D Pistol: 45.5 Meters, 91.5 Meters (1x zoom, 2x zoom.)
MA5B Assault Rifle: 38 Meters
M90 Shotgun: 22.7 Meters
S2 AM Sniper Rifle: N/A, 106.5 Meters, 427 Meters
M41 SSR Rocket Launcher: N/A, N/A
Plasma Rifle: 38 Meters
Plasma Pistol: 30.5 Meters
Needler: 38 Meters
LAAG: 29.5 Meters
Scorpion Tank: 39 Meters
Ghost: 29.5 Meters
Banshee: 29.5 Meters
Shade Stationary Gun: 29.5 Meters
M6D Pistol: No extra info.
MA5B Assault Rifle: Again, short bursts. Trust me, it's better that way.
M90 Shotgun: Fire at will. NOOOO! NOT AT WILLIAM!!!
S2 AM Sniper Rifle: The reticule never turns red when you aren't zoomed. Good thing, too, because it's way too erratic when fired from the hip.
M41 SSR Rocket Launcher: The targeting reticule never turns red. You were expecting... Bugs Bunny?
Plasma Rifle: No extra info.
Plasma Pistol: No extra info.
Needler: Now you can shoot as you please. It takes 7 needles for a chain reaction explosion, 14 for a double explosion, and the whole magazine for a triple explosion and 3 extra individual needle explosions. A whole magazine (the needles must be on the torso) is lethal even against 400% health and an overshield. Remember: Hold down the trigger until you see yourself begin to reload.
LAAG: No extra info.
Scorpion Tank: Now you can use the secondary cannon, as well. Be careful of the tank's massive dead zone.
Ghost: Shoot at them, or run them over. Or both. Stupid tip of the day: If you melee at just the right moment, an oncoming ghost can be stopped for an instant... Even an instant could save your life.
Banshee: A nice distance for both FRC and main gun.
Shade Stationary Gun: Untested. Since the plasma dischargers are identical to the other mounted guns, I believe that the reticule will turn red at the same distance.
it's good to know the size of the group of enemies you can hurt/kill, or how close you have to be to hurt yourself.
M9 HE-DP Frag Grenade: 5.9 Meters
(Pistol) 12.7mm S-AP, HE Round: 0.15 Meters
(Rocket Launcher) 102mm SC, HE Rocket: 6 Meters
Scorpion Main Cannon 11 Meters
Plasma Pistol - Overcharge: N/A
Needler: 0.3 Meters, 3.2 Meters (Individual, Chain Reaction.)
Plasma Grenade: 5.9 Meters
Fuel Rod Cannon: 3.5 Meters
Wraith Mortar: 9 Meters.
M9 HE-DP Frag Grenade: No extra info.
(Pistol) 12.7mm S-AP, HE Round: Like I said earlier, these rounds explode.
(Rocket Launcher) 102mm SC, HE Rocket: Aim for the feet. Please.
Scorpion Main Cannon No extra info.
Plasma Pistol - Overcharge: It appears to be an explosion, but it is not.
Needler: It's easier to kill someone from 3.5 meters. It's sloppy when you do it within the blast radius.
Plasma Grenade: Please don't be an idiot, and let your target give you the "hug of death". Toss, then run like hell.
Fuel Rod Cannon: Untested estimate.
Wraith Mortar: Untested estimate.
Movement Due To Explosions
This is useful, because you can use an explosive to knock your target off a ledge. Or cliff.
M9 HE-DP Frag Grenade: 1.25 Meters
M41 SSR Rocket Launcher: 3 Meters
Needler: 0.5 Meters
Plasma Grenade: 1.25 Meters
Fuel Rod Cannon: 2 Meters
Scorpion Main Cannon ?
Wraith Mortar: ?
M9 HE-DP Frag Grenade: No extra info.
M41 SSR Rocket Launcher: You can use the rocket launcher to jump higher. Put the health at 400%, point the launcher at your feet, and pull the trigger at the same time you jump. Move yourself somehow, and you can get yourself onto ledges that are 5 meters high. My favorite sight is the horizon from on top of Damnation.
Needler: No extra info.
Plasma Grenade: No extra info.
Fuel Rod Cannon: Untested estimate.
Scorpion Main Cannon Cannot be given a specific number, because you moved further away from the blast, the closer you got to the center of the blast. It confused me.
Wraith Mortar: Untested estimate. Same as the Scorpion Main Cannon.
If you feel like it, you can smack 'em. A smack on the back seems to satisfy the most. Or, in the face on no-shield-400%-health multiplayer, until the target dies... "THANK YOU SIR, MAY I HAVE ANOTHER?"
M6D Pistol: 3.2 Meters
MA5B Assault Rifle: 3.4 Meters
M90 Shotgun: 3.4 Meters
S2 AM Sniper Rifle: 3.4 Meters
M41 SSR Rocket Launcher: 3.4 Meters
Plasma Rifle: 3.3 Meters
Plasma Pistol: 3.4 Meters
Needler: 3.2 Meters
Addendum KP was right. I was off by .2 meters. This means that MC's arms are .2 Meters from the center of his body. He was also right that the noise I heard was the weapon the target was holding. I was smacking the weapon, not the other MC.
Most of the time, if you melee, the opponent is unaware that you are behind them. You don't need to get right up behind them, because all the guns reach to about 10 feet. Far-fetched, you say? Try it. Turn on "kill in order" and "Nav Points" for objectives indicator. Stand behind the target MC, at exactly .1 meter out of melee range, and melee. It won't work. Move .1 meter closer, and melee now. See? You can even look at it with a 3rd player. The melee didn't even connect, but the target still acts like it was hit...
To hear MC's heart beat, take his health down to one bar. It only beats when he has just one bar.
The overshield lasts 45 seconds per layer, on multiplayer. That's 90 seconds of a little extra protection, folks.
Active Camo last 45 seconds. Maybe longer on campaign mode. (Don't waste your time, Louis XD)
The flashlight lasts 2 minutes. It recharges in 30 seconds.
When you weapon juggle, look a little bit up. You throw your weapon further. The farthest you can throw your weapon is 4 Meters.
MC is 2 Meters tall. When you crouch, you become a mere half-meter shorter. When you look all the way down, you are actually looking at your left foot, or in the general area of your left foot.
The highest you can jump, the normal way, is 2 meters. With a rocket launcher, you can make that 5 meters. The farthest you can jump is 4 meters, the natural way. With a rocket launcher, you can jump 18 meters. This might be useful.
Crouching at the bottom of a long drop makes you take alot less damage. If you just let yourself fall, you drop down to one knee, and get up a moment later.
Crouching near the top of a jump raises your legs into these crazy-ass splits, but you can increase your jump height by a half a meter this way.
A body dissappears in multiplayer after 30 seconds. Restart this timer by throwing a grenade near it.
Throwing grenades at a light vehicle throws it for a distance. The warthog goes the farthest on one grenade, then the ghost (yeah, suprized me too...), then the tank. The tank barely jumps half a meter.
B-B-B-B-B-THAT'S ALL, FOLKS!