I just submitted a change that lets you get the center of mass from blueprint. In the case of a welded body it will give you the center of mass of the entire welded object. It should be in 4.6, but you can grab it off github now if you’d like:
https://github.com/EpicGames/UnrealEngine/commit/e5c9fb6a9695eb6c3954ccf41d89499d7fc14636