You can tighen the elbow joint, but I believe the shoulder is just a pin, so there's no way to prevent his arm from drooping under the weight. That is, don't expect to pull any poses with his arm fully extended.
And, though I don't have the mold at all or, obviously, the city/shadow commander set, I dunno taht I'd pose him with the gun up at all, that much weight is likely to wear on the joint pretty bad over time.
Theoretically the joint wouldn't wear out unless it was constantly loaded and unloaded. Creep could be a factor though...
does the shadow commander set just contain the trailer portion or is it the whole thing? Nemesis Prime and Shadow Commander trailer?
I've got mine holding onto the handle like he should, arm partially bent, and the back resting on his shoulder just slightly enough to keep it properly posed. you can do it man!
I pose mine with the BFG resting on his torso/waist, it's not noticable from the angle I display him at. (He's in truck mode just now transporting BS and SW).
Sure there is. Crack open his shoulder (it may or may not be glued tight, but a pair of pliers can split it open pretty easily. Don't worry if the pegs break, they're not required), and apply the joint tightener of choice to the shoulder post.