"There is little doubt that guns, bows, knives, and automobiles are penis extensions (for the Freudian-inclined). There's also little doubt that that is perfectly acceptable, since they are merely projections of male sexuality. They are only "wrong" if male sexuality is wrong.
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They are shaped this way for the simple reason that this shape works well for a weapon. A vagina isn't a shape you can kill with.
Which begets a whole (!) new line of discussion.
However, most psychologists who aren't busy prescribing Ritalin for schoolkids agree that certain mechanical items -- including those I cited above -- are phallic extensions.
By the way, that also extends to tall buildings, derricks, obelisks, and the Space Shuttle, whose twin Solid Rocket Boosters, combined with the phallic shape of the External Fuel Tank, give a whole new shade to the term "thrust."