So as you know from your own CQB build, the arm armor doesn't have a shoulder part. Instead it has a curved plate attached to the outer upper arm - the shoulder area is not covered. I looked up pictures of Recon and it looks to be the same upper-arm armor as Chief, but with a different shoulder part. I'm entertaining the idea of having the Mk VI upper-arm and shoulder pieces as separate models to make it easier for finishing the surface details in the crevices under the shoulder part and also make it easier to build armor variants. I'm pretty sure I can guesstimate what the area covered by the shoulder part should look like, but then how to attach parts together after they're finished....