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BattleSpace Sourcebook, pp25-26
Autocannons:
The direct descendants of the gunpowder weapons first used on ancient Terra, autocannons use a chemical charge to propel an explosive-tipped, unguided projectile at the target. There are four types of autocannon, three of which are standard weapons used in war machines from fighters to BattleMechs. The fourth is solely a naval weapon.
The standard autocannon is a rapid-firing auto-loading weapon, with a caliber ranging from 30 to 200 millimeters. Modern-day military experts grade these weapons not by caliber, but by damage potential. For example, the damage potential of a rapid-fire, 50mm cannon may place it in a higher damage category, whereas a 200mm cannon with a slow rate of fire might be classified as a medium damage weapon.
The so-called "ultra" group autocannon fire at faster than normal rates, increasing potential damage. However, this extremely rapid rate of fire makes the ammunition feed more likely to jam and reduces accuracy. The ultra-rapid fire mode means that in many cases, fewer than half the shells fired actually hit the target. To compensate for this problem, many ultra cannons are designed to fire at the standard rate as well, allowing the operator to switch to ultra mode when presented easier targets.
The LB-X series of autocannon are similar to standard cannon, but may also fire a "shotgun" shell. The larger, lighter shell increases the likelihood of hitting the target, but does less damage than the heavier standard projectile.
The fourth class of autocannon, found only on WarShips and space stations, weighs between two and five thousand tons. Though the chemical propellants they use limit their range to that of conventional autocannons, naval autocannon (NAC) can do tremendous damage by virtue of their colossal size. One or two shells alone may destroy a DropShip. However, like most naval weapons, their size also prevents them from accurately tracking small, swift targets.

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