hey i think is the rig or the model have a problem because your rig just have a one skeleton that attach to your custom arms or your model must be in the arm group
I made a tutorial on gun animations but I made it so you can animate it inside of blender. I’m sure it works for the default Roblox Animation Plugin but I strongly suggest using blender to animate it so blender can smooth down the keyframes for you.
So, I decided to remake your entire model and see if it works for me and it works perfectly fine for me. I was able to rotate and move the gun correctly and everything came with it.
That should help you a bit but if you have any questions on how some of these mechanics work, feel free to ask me and I will try my best to explain them in full details.