Imperial Ordnance

Infantry win firefights, tanks win battles, artillery wins wars.

That's how the saying goes. And it's true. Without the big guns you are doomed to be murdered gruesomely for the lulz of it, get eaten or have your soul used as a condom for all eternity. The Imperium knows this and has devised a truly staggering amount of heavy guns dedicated to ruining your shit into next Sunday from anywhere in between half a mile to half a continent away.

Man-portable weapons

The lightest of indirect support weapons can be carried by individual or paired soldiers. While this makes them tactically more flexible, they have reduced firepower compared to their heavier counterparts.

Grenade Launcher

Grenade Launcher

The grenade launcher is a light, man-portable weapon capable of firing small explosive charges. It is primarily employed by the Imperial Guard, with almost every trooper squad featuring at least one of them. It has a choice of two different payloads, anti infantry Fragmentation grenades, and anti-armour Krak grenades.

Space Marines scout bikers utilise a variation of this weapon, the "Astartes pattern" Grenade Launcher. It fires the same rounds, but with an increased rate of fire.

Mortar

A Mortar is an artillery weapon that launches shells in a high arc. They tend to have relatively short barrels and fire shells at relatively low velocities. The Imperial Guard uses them in two sizes: the standard man-portable mortar, carried and operated by two Imperial Guard troopers, and the heavy mortar, which can be used in static or towed artillery mounts, or mounted in a Griffon Heavy Mortar Carrier. They can destroy light infantry and light vehicles easily enough, but are unlikely to trouble heavy infantry or vehicles with tank grade armour.

Tank Guns

These guns are too large to be carried by infantry (excluding "heavy" weapons such as lascannons and autocannons), but are still small enough that a heavy vehicle, like the Leman Russ Battle Tank, can carry and fire them while in the thick of battle.

Battle Cannon

The primary weapon of the Leman Russ Battle Tank, it is the most commonly used tank gun for good reason: the Battle Cannon can pull double duty as both an anti-tank and anti-infantry weapon thanks to its considerable range (72"), strength (S8 AP3 = instant death to MEQs and capable of damaging any vehicle) and explosion size (5" blast = blob-killer). The cannon proved so popular that it was made the primary armament of the Macharius and Malcador super-heavy tanks, and has been forked into several versions by the Adeptus Mechanicus:

Battle Cannon (Leman Russ)

Range	Type	S	AP	Special Rules
72	Ordnance	8	3	Blast (5")

8-th edition

Since the revamping of the AP-system and moving away from "blast template" mechanics, the stats for battle cannons were changed, becoming something of this
Battle Cannon (Leman Russ)
Range=72", S=8, AP-2, Type=Heavy D6, D=D3
It is worth noting that while AP-2 now allowes 3+ and 4+ armour to actually provide at least some save-roll (5+ save for 3+ armour and 6+ save for 4+ armour), 2+ armour (like that of the Terminators) now only get 4+ save instead of 2+.

Conqueror Battle Cannon

Fitted to the Leman Russ Conqueror, this is essentially a lighter and smaller version of the Battle Cannon. Its shorter range and smaller blast radius is more than compensated by the reduced recoil and higher rate of fire, allowing for an even greater damage output than a Battle Cannon. On the tabletop, it's a "Heavy" weapon, as opposed to "Ordnance", which allows the carrying vehicle to fire its sponson weapons at full ballistic skill (though, since the Conqueror is not a heavy tank, that only applies if it holds still...).

Macharius Battle Cannon

The Macharius Heavy Tank has a twin-barreled variant of the Battle Cannon, because it has a bigger turret and because two barrels are better than one. It used to actually fire two shots, but that's been changed to a single massive (7") blast in more recent publications. Maybe they will get around to making a single massive cannon for it, too.

Vanquisher Battle Cannon

Huge gun, huge range, huge barrel, huge damage. The Leman Russ Vanquisher has its weapon tooled for only one purpose: to fuck enemy tanks and monsters hard using huge shells. It sacrifices its blast radius for an improved capacity for armor penetration (AP2, S8 + 2d6 = 10-20 with an average of 15 = roll for the circumference of the hole that is about to get punched through your armor). The Macharius can carry twin-linked Vanquisher cannons, and has enough internal space that it can also fire high-explosive shells like the regular Macharius Battle Cannon, so it's like two tanks in one -- and it costs about as much as regular Russ and a Vanquisher put together.
Worth noting, that the cannon is now somewhat nerfed, being given only S=8,AP-3 while retaining Heavy 1 and gaining D=D6R (the same rule as meltagun, abeit not only when in half-range or less).

The only problem is that the Adeptus Mechanicus lost Tigrus, the only Forge World that made Vanquisher cannons, to the Orks in M35 (somehow, you'd think the Imperium would throw so much firepower in defense of such a vital world that nothing could take it), so the only way to make new Vanquisher tanks is to take the cannons off of tanks that have been immobilized -- not a simple feat, given that the cannon is so dangerous to armor and stands out as a priority target for enemies. Two other Forge Worlds, Stygies VIII and Gryphonne IV have stepped up to try and match the old cannons (read: Forge World, the company, has produced two alternate turrets for Leman Russ Vanquishers), but they're just not quite there (though it makes no difference on the tabletop, of course). And then Tyranids ate Gryphonne. (It was delicious.)

And it also seems, that as per 6-th edition Astra Militarum Codex and onwards, the fluff was changed to do away with "Vanquisher cannon being rare" altogether - now the Leman Russ Vanquisher is noted as being simply the dedicated anti-armour varian of Leman Russ, often employed as command tank in armoured formations. However, it now also seem that Vanquisher cannon now lost it's ability to hire HE rounds, being able to only fire anit-armour APFSDS rounds, ineffective against infantry formations.

Demolisher Cannon

The Big One. Fitted onto the Leman Russ Demolisher and packing enough heat to turn Terminators and heavy and super-heavy vehicles inside-out with a single shell, the Demolisher Cannon's short range (24" -- shorter than some infantry weapons) is the only thing that stops it from being a truly amazing weapon. Even then, it can cause considerable damage (S10 AP2 Large Blast = kiss your everything goodbye) and because it is smaller than the other cannons, is one of the few ordnance weapons used by the Space Marines, mounted on the Vindicator and Land Raider Ares, and it is used as secondary armament on certain super-heavy tanks, like Baneblades and the Malcador.

Eradicator Nova Cannon

The Eradicator Nova Cannon fires shells with an unstable "sub-atomic" core (yes, this just might be a tank cannon that fires mini-nukes). These shells are somewhat hazardous to the tank and crew, as they have a non-zero probability of exploding if they get jostled while loading, so most regiments that use Leman Russ Eradicators have a margin for crew losses. They keep using the Nova cannon, though, because the shell's massive blast completely flattens any cover, so unless the targets are power armored, there's no escape for them (S6 AP4 = Instant Death for most standard infantry; less effective for beefier troops). Why a shell that detonates if it's shaken does not go off when it's fired from a freaking cannon is not explained.

Super-Heavy Tank Guns

Technically, these guns can be carried and fired by a vehicle in direct combat, but because that vehicle has to be the size of a Baneblade, they deserve a category of their own.

Accelerator Cannon

The main gun of the Fellblade. It fired armor-piercing explosive self-guided "darts" via either magnetic acceleration or gravity impellers. It was also twin-linked, though the Fellblade had severely limited ammunition for it, relying mostly on its other weapons until a suitable target presented itself. It was made on the Forge World of Tigrus (home of the original Vanquisher cannon -- which resembles the Accelerator Cannon quite closely, with the long barrel and large muzzle brake). It can fire high-explosive shells comparable to a super-charged Battle Cannon (S8 AP3 7" Blast) or armor-piercing shells comparable to what would later become the Vanquisher battle cannon (S9 AP2 3" Blast, rolling 2d6 for armor penetration).

Baneblade Cannon

The Baneblade's primary cannon is an upscaled Battle Cannon, improved in strength, armor penetration, and blast radius (S9 AP2 10" blast = 10" wide lascannon pie-plate). Its special shells are not propelled by an explosion, but are rather miniature rockets -- a feature that "counterfeit" Baneblades produced on other Forge Worlds are unable to reproduce, and so they substitute standard battle cannon rounds for them. There's no indication that they're much, if at all, different, though.

Dreadhammer Cannon

Back in the Great Crusade, Perturabo asked the Adeptus Mechanicus to build him a new cannon that could level buildings with a single shell and be carried at a reasonable speed. Because the Horus Heresy hadn't happened yet, and they were thus still feeling like innovating now and then, the tech-priests agreed, and built him the Dreadhammer. It fires shells that weigh several tons, and experiences so much recoil that even the super-heavy Typhon Heavy Siege Tank has to limit the propellant used (and thus the range) if it wants to fire on the move. At first, people made fun of Perturabo for asking for (and getting) a super-Demolisher, but it has a bigger blast radius and, if the Typhon stays still, a longer range, and it really does shake buildings to their foundations. The appearance, name, and firepower of the Dreadhammer cannon suggest some connection with the Baneblade-mounted Hellhammer cannon.

Hellhammer Cannon

The Hellhammer Cannon fires shells with a building-busting sub-atomic charge (like the Eradicator Nova Cannon). The tremendous concussive explosion (S10 AP1 7" Blast) is enough to collapse a building in one shot (Ignores Cover), so crews are trained to fire at the ground floors of structures that need to come down. The short barrel and recoil compensators give the cannon a very short range (a measly 36"). As such, the Hellhammer cannon is only used on tanks that are built for urban combat, like the Hellhammer super-heavy tank.

Quake Cannon

The Quake Cannon is an odd beast; its firepower is not unusual for a weapon of its size (S9 AP3 10" Blast), but it has a longer range than any other tank gun (180", only exceeded by the Earthshaker and Colossus cannons), and is normally an arm weapon for Warlord Battle Titans. It is occasionally mounted on a Baneblade chassis to make the Banesword, essentially a super-heavy self-propelled artillery piece, primarily employed against fortifications. Also, it's ammunition is filled with the echoing shockwave of a world that was subjected to Exterminatus, and the sound it makes as it flies through the air is said to be the screams of those who died in the exterminatus that powered it.

Stormsword Siege Cannon

The Stormsword Siege Cannon fires big bunker-busting rocket-propelled shells. Unfortunately, the Adeptus Mechanicus never considered that crews might want to fire at things other than ground floors of nearby buildings, so the cannon cannot elevate very far (though, if it did elevate too far, the recoil might just rip it out of the turret). It makes up for this by being able to turn entire city blocks to dust with a single shot.

It used to be that the Stormsword just had a Hellhammer Cannon that couldn't swivel, but apparently that made them too similar, so in the sixth-edition version of Apocalypse, GW made the Hellhammer Cannon a 7" blast and left the Stormsword with the 10" pie-plate.

Tremor Cannon

Most cannons launch shells that are fuzed to explode on or shortly before impact. Not the Banehammer's Tremor Cannon: after landing it buries into the ground before it detonates. This muffles the blast (S8 AP3 is actually the lowest-strength and worst AP of all the super-heavy tank guns), but it sends a tremendous shockwave through the earth that leaves enemies not caught in the blast stumbling and slowed. The fluff describes this as liquifying the surrounding terrain. Imagine the ground under your feet giving way, suddenly having the consistency of water. By the time you've figured out what's happened, you're partially buried in the dirt and have to dig yourself out before the enemy comes and shoots you like a horse with a broken leg. And that's if you're lucky, it's entirely possible to get buried completely and suffocate to death under several feet of earth and rubble. You can see why it would be very, very bad to be on the receiving end of this weapon.

Artillery

Sometimes, the Imperial Guard can't get in range to use tanks, or could, but only at a cost that even the Departmento Munitorum would find staggering, like when they are attacking enemy fortresses. To overcome such obstacles, the Adeptus Mechanicus had a brilliant idea: make even bigger guns with a longer range. These guns are so big that they cannot be fired on the move; they are often built into static defensive positions, or for regiments that need to be more mobile, they may have guns placed on towed platforms, or on self-propelled chassis (see Imperial Ordnance Pieces for more information).

Colossus Siege Mortar

The Colossus Siege Mortar is huge, cumbersome and highly destructive, and it's so large that its shells have to be loaded by cranes instead of by hand. It fires massive concussive rounds that will turn its target to paste (as long as it's a human wearing something less than terminator armor -- S6 AP3 is good against infantry blobs, not so much for harder targets), even if they are hiding in cover. Because one cannon wasn't enough, the Imperium built the Dominus Armored Siege Bombard, a variant of the CRASSUS ARMORED ASSAULT TRANSPORT that carries an automated trio of Colossus cannons (though the Dominus only costs as much as two Colossi...which actually makes sense, as the Colossi are built on a Leman Russ chassis where-as the Dominus just has to pay for an extra cannon, not a whole new vehicle to carry it).

Earthshaker Cannon

The Earthshaker Cannon is the Battle Cannon of artillery pieces: common, highly recognizable and very lethal (S9 AP3 5" Blast anywhere within twenty feet). It fires its shots further than any weapon except for the Manticore Launcher Tank and Deathstrike Missile Launcher.

Griffon Heavy Mortar

The Griffon Heavy Mortar is the smallest and lightest of the artillery guns. Its firepower is not impressive (it's S6 AP4 like the Colossus or Eradicator, but doesn't ignore cover), and its range is also relatively short (48"), but because it is so small and light, it can sustain higher rates of fire than other guns and, when carried in the heavy chassis of a Griffon Heavy Mortar Carrier, it can be fired with more accuracy (i.e. re-roll scatter dice).

Medusa Siege Cannon

The Medusa Siege Cannon is the shortest-ranged of the Imperium's artillery weapons (at 36", only the Demolisher is shorter-ranged, though bastion breachers go 48"), but it is not meant to launch munitions over fortified walls; rather, it is meant to shoot fortifications directly and destroy them (which it certainly does, at S10 AP2 5" Blast) -- and when regular shells aren't enough, they can fire special "bastion-breacher" shells that sacrifice blast radius for even better armor penetration (AP1, S10+2d6 3" Blast).