Its probibly not the gun itself. Streamlines can't fly strait for crap. Its all because of that little hole on the tip. This can be fixed by cutting off the hollow tip and putting hot glue of some other weight in there.
It has very little to do with the "whistler" hole in the tip at the velocities a stock blaster can produce. It does come into play however, with higher-powered blasters.
With stock(ish) blasters, the problem is the balance of Streamlines. Whereas most stock darts' weight is focused in the tip of the dart(Whistlers, Suction cups, Taggers), a Streamline has very little mass there. Hasbro designed them to be low profile, yet they needed some mass in order to fly. They did this by extending the "stem" back further into the dart, making them effectively stall constantly in flight, which is the effect we know as "fishtailing."
If you purchase one of the hot glue guns with a precision tip, you can fill the heads with hot glue without modifiying the tip whatsoever. Simply fill them via through the hole, trim any excess glue that seeps out after it cools.