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I assume if clearing out hordes of lightly lightly armoured opponents and if you wanted to keep some distance, surely a rotor cannon would be preferable to a non-heavy flamer as it has better range?]]> http://www.dakkadakka.com/dakkaforum/posts/preList/818751/11816270.page http://www.dakkadakka.com/dakkaforum/posts/preList/818751/11816270.page GMT http://www.dakkadakka.com/dakkaforum/posts/preList/818751/11816273.page http://www.dakkadakka.com/dakkaforum/posts/preList/818751/11816273.page GMT 40k marine forces.

Certain weapons were selected and the Mechanicus started producing those weapons specifically to arm Marine chapters.

Every other weapon fell by the wayside and either became a chapter specific weapon or was eventually lost and replaced with a more common weapon.]]> http://www.dakkadakka.com/dakkaforum/posts/preList/818751/11816279.page http://www.dakkadakka.com/dakkaforum/posts/preList/818751/11816279.page GMT
The original 2nd edition Chaos Codex gave these same reasons for why Chaos Terminators used Reaper Autocannons instead of Assault Cannons, and for why they did not have Land Speeders or any other anti-grav vehicles any longer. The Traitor Legions could no longer count on stable logistical support and kept using the most reliable least maintenance intensive weapons they had from the time of the Great Crusade.

Similarly for the loyalist Space Marines, the shattered Imperium had to rebuild and settled on certain weapons to keep producing/maintaining and then things ossified.]]> http://www.dakkadakka.com/dakkaforum/posts/preList/818751/11816293.page http://www.dakkadakka.com/dakkaforum/posts/preList/818751/11816293.page GMT
Rotor Cannons are best employed against lightly armoured hordes, where their volume of firepower matters more than sheer stopping power.

Since the Second Founding? It’s kind of rare Astartes will have to fight in such a manner. They’ve been replaced as the premiere armed force of The Imperium by the Imperial Guard.

Yes, they will still have to tackle numberless hordes, but having shifted to essentially strike and fade tactics? The Rotor Cannon just doesn’t really offer much compared to Bolter fire, or the massed ranks of Imperial Guard firing Lasguns, because Marines can no longer deploy in the sorts of numbers where the Rotor Cannon finds it niche.]]> http://www.dakkadakka.com/dakkaforum/posts/preList/818751/11816313.page http://www.dakkadakka.com/dakkaforum/posts/preList/818751/11816313.page GMT