Any of these arms are nice pieces, and make me wonder if they would fit onto a more standard dreadnought for some conversion work. The Praetor Armoured Assault Launcher is just huge. It looks like just the launcher itself is nearly as large as a chimera.
Experimental rules for the Kheres Assault Cannon http://lyris.games-workshop.com/t/92430/7211474/5157/0/
Experimental rules for teh Praetor Armoured Assault Launcher http://lyris.games-workshop.com/t/92430/7211474/5711/0/
The ancient and prized Contemptor Dreadnought, once a mainstay of the Astartes Legions during the Great Crusade, is capable of fielding many different heavy weapons. The Contemptor Pattern Plasma Cannon and Contemptor Pattern Heavy Bolter are both precursors to the weapons systems found on the MkIV and MkV Dreadnoughts, and the Kheres Pattern Assault Cannon is considered by some techno-savants to be the forerunner to the Assault Cannon carried by Space Marine Terminators.