The Big Guns
The Imperial Navy employs some of the most advanced weapons known to mankind, derivatives of dark age technology and post-dark age Adeptus Mechanicus ingenuity. Dark Age ships never needed to overcome void-shields are rebellion among machines was unheard of. Yet in the Imperium rebels are a constant threat, and the Adeptus Mechanicus had to figure a way to overcome void shields, which is no small task. The final battleships of the Imperium are a mixture of these two strands of thought, to yield a truly formidable fighting platform enabling the Imperium to project its might anywhere in the known galaxy.
Ship to Ship
Combat between main battle groups in 40K has developed into a close up and brutal fight due to the powerful shield technologies employed. Whereas un-shielded ships often engage with drones and lance at ranges of hundreds or even thousands on Kilometres, the great battleships can only be taken down with point blank broadside. It is ironic that in 40K much of the massive battleship vs battleship combat between loyal and rebel human forces resemble Man-o'-War tactics 16th-19th centuries, and the larger ships using line of battle tactics . Indeed, many high ranking naval officers often refer to their vessels as Man-o'-War.
It is important to remember that while a Man-o'-War can only be taken down with broadsides, a battleship's lances have massive range and can engage enemies thousands of Kilometres away. Small ships stand no chance against a battleship and will be taken down in seconds once detected, and often well before the small ship is aware the battle ship is in the area. This gives the Imperial Navy a powerful psychological warfare tool in the fight against rebels and pirates. It is a simple fact that a single Man-o'-War can clear out en entire solar system of pirates in minutes. Such is their power.
Round Shot
Basically slow moving cannon balls made of solid iron! These are not primary ship killing weapons but are used en masse to make 'windows' in the conversion field layer of Void Shields so that the serious weapons can be brought to bare (Lances). Round shot is delivered via mag-lev accelerators in broadsides (old ship of the line style). Round shot is very slow, often traveling at less the 100-500kph (one hundred). Impact wise they can crush outer ablative armour (some advanced Void Shields are a power-field layer with can soften the front of round shot to reduce damage).
Cannon: Round shot have huge mass and this causes problems with firing in zero-g of space. Cannon are designed to fire and not place forces on the ship. There are free floating tubes inside a mag(netic) sleeve running the breadth of the ship, stabilized on the opposite side with a ring thrusters. Cannon can fire either direction, and as they are loaded in the middle of the tube they can fire simultaneously in both directions if need be (thrusters aren't used to counteract acceleration forces during a simultaneous dual fire but merely to 'trim'). The firing tubes can move around quite a bit in their mag-sleeve.
Cannon power is directly proportional to the length of the tube (as military grade weapons use the standard mag-lev cannon ring linked in series). The longer the tube the more acceleration time. The acceleration is constant and standard thruster rings will counter the forces no matter the length.
Assault Pods
Use the same launch system as round shot.
Lances
OMG Crazy Dark Age technology. P-Field emitter based particle beam weapon. Fires a beam screaming through border-space until it encounters the shadow of a dense concentration of matter fields (internal atomic forces) which causes the beam to re-phase. The re-phased particles are travelling at around one third the light speed (which is virtually impossible) within the target which is ripped to sheds from the inside out. Lances on Imperial ships are usually powerful enough to cut though ablative armour and superstructure down to shield generators within 6 seconds (average time the warp drop field is down once overloaded). This power level means they can cut a ship in half (of similar size) in around 1-2 minute. A ship cut in half is considered a TK or 'Total Kill'.
Plasma Lance/ Plasma Torpedo
In modern terminology this is a 'shaped charge' only using far more powerful explosives than that used in modern day warfare. The torpedo detonates just before impact. The resulting explosion drives the liner into the target as a jet of super heated metal. The metal jet produced by these torpedoes is so hot that it becomes as true plasma with a trailing hyper-velocity 'spear' of molten metal. This is sufficient to punch through even the thickest of armours and is also effective against heat resistant ceramic based composites and derivatives.
Note: Often used to clear an area of a large ship (Battle Ship at 3-10km) or power fields and defence grid laz-cannon. Much like a 'daisy cutter' clears trees the plasma torpedoes clear defences.
Orbital Bombardment
Orbital Superiority is a core doctrine of the Navy, and serves as a gateway to the deployment of the Imperial Guard.
Vacuum bombs
These potent thermobaric weapons are used on worlds where atmospheric oxygen is present. These bombs initially detonate just before landing, ripping open the casing and ejecting a huge cloud of mist/ vapour/ dust of compound into the air. The compound mixes in the air, and uses the oxygen within the air as it's oxidizer. Once dispersed the second charge detonates the resulting compound/ air mixture. As the oxidizer is not contained within the weapon, the resulting explosion is far larger for the size of weapon than if conventional explosives are used.
Cyclotron Bombs
These use Cyclotronic Radiation.
Plasma Mine (anti-shipping)
At the centre of this mine is a super powerful microwave emitter. Normally microwaves are deflected by metallic objects (such as a ships hull) and they take little damage. However using the surrounding water as a medium, the mine detonates when in proximity to a ship releasing a Massive Microwave Pulse or MMP. This MMP flash heats the surrounding water into a boiling plasma, and it is the plasma that damages the ship. The plasma cuts through the hull of the ship or submarine with ease, going through several skin layers, leaving a wide and usually perfectly circular hole.
Star Bombs
These are super 'Nukes'. All fuel matter is directly converted to energy via high-yield pulsed conversion beam to create gigaton (one billion tons of TNT) explosions (largest modern Nuke ever is 50 megatons, most top out at 20, a 1 gigaton star bomb is 1,000 megatons). The Star Bombs at 1 gigaton will measure 8.0 on the Richter scale and will shift faults.
Crust Buster [ELE]
Shaped charge built of P-field enhanced star bombs (nukes). The size of of sky-scrapper (think the Chrysler Building), designed to punch through tectonic plates to release masses of magma (super-volcano). It is in the Teratons (10 on the Richter scale). Primarily an anti-Ecorium (Ecopolis/ Hive World) weapon and used to destroy massive underground bunkers complexes on a continent scale. Requires Adeptus Mechanicus direct sanction.
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