I'm not going to lie - this gun rocks. It can kill hordes thanks to its 10" blast. It can kill MEQs due to Str10. It can kill Terminators due to AP1. And it will absolutely ruin anyone in a tank's day with 3D6+10AP. Even under the blast it deals out 2D6+10AP, regardless of range!
Using this gun is quite simple, point it at something and watch it die. Tightly grouped transports and heavy tanks will all crumble (AP1 remember!). The catch is to not point it anywhere near your own forces - one bad scatter and you'll be scraping barbequed Skitarii off the pavement for weeks.
An interesting peculiarity of this weapon is that it is Ordnance Melta - so technically you should be able to roll each AP die twice and pick the highest result (as per the Ordnance AP rules). This would make it ludicrously good at getting through armour (roll two blue dice, roll two red dice, add the highest red and blue to 10 and compare to the vehicles AV). However the rules for the gun have an explicit explaination of how to work out AP, not mentioning the Ordnance classification of the weapon. I personally think it comes down to how much you like your opponent as to which rules you should bat for (or in seriousness I think ignoring Ordnance re-roll is probably the best idea).
The other downside is that as a single blast any eternal warriors, MCs, and GCs will only be taking one wound regardless. 72" range is ok, but nothing too spectacular.
The ideal situalion is a Land Raider with rhinos close by (or Leman Russ and Chimeras). Target the LR, to use the 3D6 on the AV14 vehicle. The massive blast will clip several of the transports and have a very good chance of busting them open. One blast can potentially lead to alot or wrecked vehicles and infantry in the open (a death sentence in Apoc).
The MTC is a great all-rounder. It can clean up infantry (and lots of them), vehicles, and has a good chance to damage other SHs. It's only weak point is high toughness, multi-wound creatures, so if facing an all carnifex army load up with something different, but against anything else somebody's going to get a hurnting.
