Hello Titanseers, welcome back, thanks for dropping by. Since the Reaver Titan was released around about 2006 by Forge World they have consistently failed to provide a number of options for the Titans, there was the Turbo Laser Redoubt which was a proto-Carapace design and abandoned and made into a Redoubt Bunker instead. Once Adeptus Titanicus was released with several carapace options for the Reaver, there was a glimmer of hope more carapace options would come available - alas none have come and some arm weapon options have been removed (Chainfist). So some people have retro-designed based on the AT variants and scaled up to produce 3D Prints. This is such a weapon which came with a recent 2nd-hand Reaver purchase - the mighty Vulcan Mega-Bolter (VMB).
This sculpt is slightly (10%) too small and the carapace plug is moderately (25%) to large for the Reaver and considerably (50%) too shallow to fit the FW Reaver, however it is passable and needs must. Once I get a 3D Printer up and running, maybe I will upscale the .STL files and adjust the fitting-plug.
I painted this to match the same as my converted Carapace Vulcan Mega Bolter
I quite like the low profile this one has with the side-slung ammo hoppers. The advantage of VMB over a Apocalypse Missile Launcher is six shots and possibility of more with sixes. So on a Close Combat orientated Engine this may be more appropriate, besides I like the 'dakka dakka' this represents! 
Here is a sneak preview of the 2nd-hand Reaver, was Legio Defensor, will obviously be re-painted to Legio Astorum!

Cheers, Siph