Whilst technically not quite a full shouldering arms pose (since the fist is still on the trigger rather than the base of the weapon), this marine has his meltagun held vertically in his right hand while pointing at a distant target with his left.
The conversion here uses a metal meltagun that I purchased a long time ago in the old style clam packs. The only real way to ensure structural soundness of this model is to pin the metal to the plastic. This takes some significant drilling work -- and with a sound drill that isn't likely to snap as pressure is applied. But the lead alloy that the melta gun in made out of is reasonably soft, and so can give under the pressure of a hand drill. That said, some care is needed to not apply too much pressure as the drill bit can break under high pressure (as I've discovered on past occasions). The pin in this model runs deep in to the metal fist and in to the plastic arm, and is a simple paper clip.
The left arm is a simple pointed finger glued in to place and the corvus helmet looking in the same direction to give a sense of purpose to the conversion. The back pack follows my previous meltagun / gun kata marine and is from the grey knights plastic space marine boxed set. Eventually this torso will be married up with a set of true scale legs and some accessories added to make the final product.