Non fullbody animation

Hello

I got a little problem with my animation,, The full body animations works cool, but when i try to make shot or reload animation. I want to replace the arm animation with that... it makes an addiative animation and not overwrite the original one. I deleted the other bone animations, and i made higher priority for them, but it doesn't works. Can you give me some sort of tutorial or help how should i export the animation properly? Thx a lot