Usually i got this problem in the past when i rescaled one or more bones/helpers in max after it's creation, even if it was not keyed, even if it was before skinning it to your weapon.
Don't know how, but the exporter know the bone/helper original dimensions, and export it as this dimension (cause exporter don't export scales animations), and so, when the exporter redimension your bone/helper to fit to its original size, it deform your model aswell.
That's the only answer i know for this sort of resulting problem, maybe there are others possibilities.
To solve this (if it's really that, try to remember if you scaled any bone), you MUST create new bones... and so, redo all your anims and vertex skinning.