We use a parameter called "MASS" and a post-regeneration relation of "MASS = PRO_MP_MASS".
These are two 10x10x5 blocks. The one on the left has steel assigned as a material and the one on the right has a density of 1.00 and an assigned weight of 15.00 lbs.
The assembly mass parameter shows the computed weight of the left block and the assigned weight of the right block. If I run massprops, it uses the computed weight of both, but the MASS parameter is correct. (see pictures)
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Hope this helps,
Arnold