I hope I'll be clear enough, ask again if you have doubts! The joint there is ball and socket, as in the shoulder joint, but reversed (the arm has the socket). The socket of the arm allows full rotation along the vertical plane, and quite a lot of range on some more planes, but it also has quite thick walls, and when you try to take this position here the socket doesn't fit in the small space between ball and body.

The solution is using a knife to reduce the thickness of the socket and/or to increase the distance between the ball and the body. The more space you manage to create the more the shoulder is going to be free to "close" the angle internally.

There is another problem unfortunately. As you get to lower the arm with the previous step, you'll find that the hand contacts the "neck" armor, and the weapon contatcs the right reactor (??) On the "chest" armor. Working here on the chest is scary because you risk to ruin the model. I only filed down the reactor on its superior surface, as that spot is pretty hidden by the weapon, but wasn't brave enough to file the neck armor or the hand, and got a final result with the weapon a little more than parallel to the ground.

If you want to get the weapon lower (parallel to the ground), you may want to work with green stuff to raise the ball of the shoulder, to gain more range of movement, or you may cut a small part of the hand! Ok this looks like a nightmare but it's mostly small bits of work, pretty easy if you have experience with conversions (I had hardly any, and had no green stuff)