Soulstorm

While a unit's accuracy is not actually displayed anywhere in the in-game statistics, it's a large part of everything you do that involves combat in Dawn of War; that is to say, just about everything. While knowing a unit's accuracy is, in many cases, rather unnecessary, it's helpful to have a good understanding of what affects your units' chances to hit the target in those situations where it can be manipulated by the player. There are also many units that have special rules governing their accuracy while firing on the move - knowing these before you build the units can help you decide what's worth doing, and what not to bother with. A Hellhound, for example, is far better at chasing fleeing enemy units than a Marauder, despite the Marauder's speed advantage and ability to ignore terrain, simply due to the Hellhound's massive superiority in moving accuracy.

First and foremost, let's look at unit accuracy in general. A unit's actual damage per second (DPS, the term used by most gamers to compare damage between units of any game) is a combination of four factors: the shooter's base damage, the shooter's accuracy, the target's armor class, and any external factors such as cover, energy fields, or similar things. An Imperial Guardsman's listed ranged damage is 20-24, but he only actually does 5.5 DPS to most infantry targets. If you've ever wondered why your units don't inflict anywhere near as much damage as they're supposed to according to their stat menu, now you know.

Obviously, a loss of accuracy is a bad thing, and a big one can cripple your units' damage, even your elite units. For most races, there are two main factors that can damage your accuracy - breaking squad morale, and firing on the move. The Eldar have a third way to hurt it, their Fleet of Foot ability, as well as a way to raise it, the Farseer's Guide ability.

1. Breaking squad morale:
   A squad whose morale reaches 0 or is automatically broken by a special effect or ability is "broken". This does two things - first, it makes the squad move faster, so it can get out of combat quickly. More importantly, it reduces the broken squad's accuracy to 20% of its original. Depending on what the original accuracy was, this might be a big difference (Eldar Rangers with 100% accuracy drop to 20%) or, frankly, not so much (Ork Mad Doc with 40% accuracy drops to 32%, pretty insignificant).
2. Fire on the Move (FOTM):
   FOTM accuracy is, for most units in the game, 15%. Note that it's just a straight 15% - not 15% of the original value, like broken morale, so Ork Shootas (60% accuracy) and Imperial Guardsmen (50% accuracy) both have 15% FOTM accuracy. There are some special units that have different accuracy rates while moving - these will be covered at the end of this tip.
3. Fleet of Foot (FoF):
   While FoF is activated, Eldar units have only 50% their original accuracy.
4. Guide:
   The Farseer may cast Guide on any friendly unit she isn't attached to. Guide increases the accuracy of the unit by 30% for 25 seconds. This can be used to either supplement a normal squad in combat, or alternatively it can be cast on a broken squad to bump their accuracy from 20% to 50% (Warp Spiders, for example, only have 65% accuracy under normal circumstances - casting Guide on a broken Spider squad will bump them up to 50%, thus making them almost as effective as an unbroken squad and still dealing most of their regular damage). Another sneaky trick is to cast Guide on a Fire Dragon squad when chasing a fleeing vehicle - it increases their FOTM accuracy to 45%, only 20% worse than their usual, allowing you to run and still fire effectively at the fleeing tank. Do not, however, cast Guide on Rangers under normal circumstances. They already have 100% accuracy so the extra 30% has no effect on them.

Many units in the game have special accuracy rates when firing on the move. First, note that all close combat attacks must be made while stationary, so melee blows cannot be struck while the fighter is moving. Any weapon with a setup time, of course, cannot fire on the move at all. All flamer-style weapons, including regular flamers, Ork burnas, and vehicle-mounted flamers such as Defiler heavy flamers and the Immolator's twin heavy flamers, have a 70% FOTM accuracy, only a 30% drop, making them excellent for "chaser" squads to hound running enemies or to fire while charging into melee. Let's look at the rest of them, sorted by race.

• Necron Warriors - 64% FOTM
• Necron Restored Monolith - No penalty

• Imperial Guard Hellhound Inferno Cannon - 65%
• Imperial Guard Marauder Heavy Bolters - 5%

• Space Marine Grey Knight Storm Bolters - 10%
• Space Marine Terminator Storm Bolters - 10%
• Space Marine Land Speeder Heavy Bolters - 10%

• Tau Barracuda Missile Pods - 25%
• Tau Devilfish Burst Cannon - No penalty
• Tau Skyray - No penalty

• Eldar Falcon Scatter Laser - Cannot fire on the move
• Eldar Nightwing Shuriken Catapults - Cannot fire on the move
• Eldar Nightwing anti-aircraft Bright Lance - 30%

• Chaos Hell Talon - all weapons - 30%

• Battle Sisters Celestian Meltagun - 25%
• Battle Sisters Celestian Multi-melta - Cannot fire on the move
• Battle Sisters Seraphim twin Bolt Pistols - 70%
• Battle Sisters Immolator Multimelta - 25%
• Battle Sisters "Emperor's Touch" Bolters - 50%
• Battle Sisters Lightning anti-aircraft Lascannon - 50%
• Battle Sisters Lightning Autocannon - 55%

• Dark Eldar squad leaders' Splinter Cannons (upgraded from Lab) - 10%
• Dark Eldar Archon's Splinter Cannon (upgraded from Lab) - 10%
• Dark Eldar Haemonculus Destructor - 10%
• Dark Eldar Raven Dark Lance - 50%
• Dark Eldar Incubus Tormentor Helms - 10%
• Dark Eldar Talos Stinger - 35%
• Dark Eldar Reaver Jetbike - 10%
• Dark Eldar Reaver Jetbike with Targeting Module - 55%
• Dark Eldar Raider Dark Lance - 35%

Note that the Dark Eldar squad leaders and Archon have standard 15% FOTM with their default weapons - only the upgraded cannons suffer from the 10% replacement.

As you can see, some units (the Marauder) are terrible at running down units, while others (Seraphim, Lightnings, Necron Warriors) are rather good at it. Know which one you're using before you try to chase someone down. Also remember that a unit's accuracy affects ALL of its weapons, not just its ranged ones - this is especially crucial when dealing with Eldar, as leaving FoF turned on a Banshee squad in melee combat is literally halving their effectiveness.

Thanks to the guys at Relic Wiki for providing the raw statistics!