Animations between full 3P and arms 1P skeletons

I am building a shooter game, and have ray-trace based gun firing working. So far I am using default mannequin for the 3p mesh and animations, and default mannequin arms for 1p mesh and animations. However the animations appear to be different. This makes a big problem where the guns held by the respective meshes are in different position angles. The raytrace shot does fire accurately to the right point, this is being replicated and reproduced accurately from either the 1p gun muzzle or the 3p gun muzzle. However because the muzzles start in a different point, the trace can be visually off at a weird angle instead of straight out from the gun. See pic of guns with both meshes visible:

I notice that the arms skeleton appears to still have the full hierarchy of bones, however most are hidden. Does this mean I can use the same animations to drive the full skeleton or the arms skeleton? At the moment, it is not allowing me to do it. Am I barking up the right tree here, assuming that arms and full skeleton can be driven from the same set of animations? Is there a way to derive “arms only” animation from a full animation? At the moment I am not set up to make meshes and animations, so will be using assets from Adobe Mixamo, but if I can’t even get it working with default mannequin, then I will be in trouble…
Any help much appreciated thanks!