Took a page out of Fallout 4's combat armor and edited an older Fallout 3 combat armor mod to throw over outfits. Clips like hell for most outfits, but I found Dragbody's Fallout 4 recon armor port works the best. Anyway so I did some edits to it, such as adding forearm guards and therein lies the problem; the forearm guards don't move with the rest of the arms LOL.

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It's not perfect but it's passable when the character is standing still as seen above, but moving or pulling a weapon out and they start floating. I think it might have to do with the BSDismemberSkinInstance, but I'm not completely sure. I'm far from a NifSkope master lol.

If anyone can give some advice, I'd really appreciate it!

I ran into a simaler problem like this years ago and I cannot remember the cause. My only advice would be to reweight the arms and export them seperately and then open them in nifskope and paste the arms into the armor nif. Make sure your export setting are correct.