IIRC it is because Max5 uses a newer obj version than Poser, and that's why the gun "fall apart". What I have done in these cases is to save if as a 3ds in Max, then import THAT into Poser and use Poser to convert it to a obj that can then be mapped in UVMapper. But it's a PITA if you have grouped it and all. You CAN try to reset the pivots, sometimes I have had good effect doing that.

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You just can't put the words "Poserites" and "happy" in the same sentence - didn't you know that? LaurieA
  Using Poser since 2002. Currently at Version 11.1 - Win 10.