To clear the issues up, I will aid Kalon in the aspect of rules for the said weapon, and a brief description of its effects for those who don't play
Warhammer 40,000.
Autocannon
Range:48"
Strength:7
AP:4
Type: Heavy 2.
Translation:
Range: Self explanatory. Unless the enemy model/squad is in range, you cannot shoot at it.
Strength: Each model type (Tau Firewarriors, Necron Warriors, Space Marines,etc) has a Toughness value, between 1 and 10, but for usual troopers it's usually from about 3-6. If the weapon's Strength value is twice that of the enemy model which is being shot at's Tougness value, then the said enemy, if it fails its armour save (On a six sided dice, most races need a 5+, meaning that only fives and sixes count as deflecting the incoming fire) then it is killed immediately, and not taking off the model one wound, as some models can have many wounds before it keels over and goes to visit the Emperor on his Golden Throne.
In shooting at vehicles, if a hit is scored, then a dice is rolled, and added to the weapon's strength. The autocannon can potentially therefore thwack down one of the new Chaos Defilers, and a writeup for the said Defiler will be coming shortly.
AP (Armour Piercing): If the weapon has an AP value lower than the target's armour save, then the target is wounded automatically, since in gaming terms the projectile just batters its way through the armour the target is wearing. With the autocannon, this means that broadly it can take out most basic squaddies of other armies, and certainly some of the beefier units as well.
Weapon Type: There are three types of weapon in the Warhammer 40,000 universe; Rapid Fire, Assault and Heavy.
Rapid Fire means that if the model bearing the weapon stands still, it can either shoot once at a target at its maximum Range, or twice at a target up to or below half its maximum range, and is the default weapon type for most army's basic troopers. If moving, only one shot at up to half range is permitted.
Assault weapons mean that they can be fired as many times as included in the weapon profile, even if they move or not. These sort of weapons are good for, well, assaults.
Example: I have a Storm Bolter on my Space Marine Captain in Terminator armour. I move him the maximum allowed (six inches a turn), and that puts me in range of my opponents Big Evil Thing Of Extreme Nastiness. I can now shoot at him with the full amount specified on the weapon profile (twice) but I had to move to get into range to shoot at it.
Heavy Weapons: These weapons can only fire if the model/s bearing them did not move that turn. Again, they can fire as many times on the weapon profile, be it twice or only once.If moved, unless on a vehicle, it cannot fire.
Consequently, we can see that the Autocannon is a potent weapon in the realm of Warhammer 40,000. Recommended uses are for infantry horde killing, however it will also serve as a medium vehicle killer as well.
Game on.
(Note: Details correct at time of posting.)