OK, two railguns pointed in opposite directions :)

Mars missions have to do a lengthy burn to drop into orbit. Point it the right way and you're even using less fuel.

Well, they /could/ factor the recoil into their insertion burn calculations. That's just a matter of the repeatability of the railgun's acceleration.

Of course the better way to outpace Voyager at this point is probably an ion engine, Voyager is sailing along at 17 km/s and the highest number I could find for a current railgun is 2.4-3 km/s (and these way waaay more than 300kg), that leaves the orbiter with >14km/s of the velocity required to beat Voyager. Each mission will come in at different orbital velocities but I found that Odyssey had a velocity of "5.907 kilometers per second" [1] before it's orbital insertion burn. Missions are more inclined to come in at or around their final orbital velocity (getting there faster requires more fuel for braking into orbit and more weight dedicated to that fuel requiring more fuel [repeat until your entire launch weight is fuel with a postage stamp as your probe]).

[1] http://mars.jpl.nasa.gov/odyssey/mission/timeline/mtmoi/moid...

Step 1: Fire railgun.

Step 2: Eject railgun.