The Onslaught Gatling Cannon is a powerful Imperial rotary ballistic weapon capable of firing thousands of rounds per solar minute.
Heavy Onslaught Gatling Cannon
A Heavy Onslaught Gatling Cannon in use by a Redemptor Dreadnought
A Heavy Onslaught Gatling Cannon is a variant of the standard Onslaught Gatling Cannon that has been sized to fit a Redemptor Dreadnought and a Repulsor armoured transport.
One jointed arm of the Redemptor is given over to the devastating Heavy Onslaught Gatling Cannon -- a rotary ballistic weapon so powerful that it can chew through a band of Heretic Astartes in a single pass. The Heavy Onslaught Gatling Cannon can also serve as the primary weapon of the Repulsor.
Sources
Codex Adeptus Astartes - Space Marines (8th Edition), pp. 83, 87, 166, 182