This might not be the problem in your case, but I once encountered similar behaviour when parenting an object with children to a moving object. In my case, some of the children developed offsets when the parent was rotated (this manifested as variable-width gaps appearing between the various child objects).
I was able to fix the problem by altering all the children such that their pivot points were coincident with the pivot point of the object containing them. In your case, this would mean that the pivot points of the `Slide`, `Receiver`, and `Grip` objects would need to be located such that each of these objects is properly positioned when placed at (0, 0, 0) in the local coordinate system of the `Gun` object.
I suspect this has something to do with [floating point error][1]. It seems that the position of a child object in the local coordinate system of a rotating object isn't preserved particularly well during rotation.
[1]: http://en.wikipedia.org/wiki/Floating_point#Accuracy_problems
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