It may have to do with the welding. The sword+scabbard geometry probably has different vertex numbers for the vertices that should connect to the straps, which could mess up the weld statements.

If you want to be able to load a variety of swords into those scabbards, you're probably better off with parented props.

The pen is mightier than the sword. But if you literally want to have some impact, use a typewriter

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