You can check out the Raptor sample in Spine to see the setup. The gun is actually in there twice. Once attached to the "gun" bone on the hip of the skeleton. This is the standard position. Then the gun is also an attachment on the fronthand, and invisible by default. When Spineboy grabs the gun, the gun attachment on the "gun" bone is set to be invisible and the gun attachment on the fronthand bone is set to visible.