legal import types

I am curious as to what the government considers as "suitable for sporting purposes" in respect to whats legal to import. Is there a list of components which can not be installed on a firearm in order for it to be imported? is there a list of firearms that are generally recognized as being importable as a whole? I know that there is no problem in the importation of Benelli M1S90's with pistol grips, but I am suprised that it was allowed to be imported because they usually consider a pistol grip a evil feature..so where is it written that it can be imported?