Something I figured out today and thought to both share and note so I don't forget it. Often when you weight copy a piece in Blender the piece ends up getting too much vertice information and stretches and bends all over the place. My normal resolve is to delete any part of the model in Blender that has a vertice assignment that I don't want passed on the piece. That's important when it comes to say a costume piece on the upperarm as that position is just too close to the clavicle. However when you say try and burn a large, long or tall piece it can still come out a mess. In todays this was a sword. Weighting it to just the hand doesn't work because the blade then bends and stretches everywhere. The fix is the Shrink Trick. In Blender first go into Edit mode and shrink that piece to make it tiny then fit it inside the part of mesh in question, in this example the hand. Then weight copy. Then before or after joining the new piece to the armature, doesn't matter, rescale the piece back to size. In my example I made a duplicate of it before shrinking it so I would have a model of the size and position it needed to come back to. Anyways works like a charm, sword no longer bends all over the place yet follows the hand animation all the same.