This one is a hard one to catch, but I noticed when after opening my inventory when running. Although it's obviously not a huge deal because you can't see it outside of FP perspective, but it is a bug nonetheless.

I'm guessing the modeling with holding guns interferes with it somehow.

I wanted to use this mod with the First Person Model mod but it looks ridiculous without my arms moving lol.

Mods are just not compatible, simple as that.

Well, this is the case with no other mod installed, and I don't think it's supposed to be like that. I just used the First Person Mod as an example of why it could be an issue.