Thudd gun
From Lexicanum
Thudd Guns were 1st and 2nd Edition support weapons. It shoots rapidly, its multiple barrels pumping in and out as each shell fires in succession with a distinctive "thud-thud-thud-thud" noise, leading to the common name of thudd gun.
Officially they are known as quad launchers. They have a high rate of fire due to simultaneous reloading and it can also be moved easily due to being mounted on a small robotic tractor unit. It requires only one trooper to move and fire it and it is so simple that it doesn't technically require a dedicated gunner, anyone can fire a Thudd Gun if the designated gunner is killed.
They are ancient weapons, employed during the Horus Heresy, and even being used within the Imperial Palace on Terra to defend strategic corridors against the attacking Traitors. More recently, they have been employed in the Badab War as well as the Tyrannic Wars.1-p92
Thudd guns were reintroduced in Forge World's Imperial Armour, where they are known as quad launchers. Forge World's quad launchers are themselves immobile, lacking the robotic tractor unit, but are able to take a Centaur Carrier vehicle as a tow, and unless the weapon is towed it can't be moved.2
