The SRS99 from Halo is probably the most scientifically sound and hardcore sniper rifle in any video game. I've put it in the hands of many models over the years and it never ceases to look intimidating and ultra lethal. It was clearly designed by someone who understands why science fiction is a separate genre from fantasy(or at least understood it while he/she made this weapon. Clearly this thought was lost when the other UNSC weapons were created). A great sci fi design is one that has a strong real world influence. In this case, that would be the NTW-20. Even the ammunition it fires is real to an extent. Take the 14.5x114mm round used by the PTRD in WWII for taking down tanks and turn it into an APFSDS(modern tanks fire much larger versions of this) and you've got a projectile capable of punching through most armor or just completely eradicating a softer target with a solid center mass hit(good thing the Geneva Conventions don't say shit about genocidal aliens).
Anyways, here's Sarah Kerrigan about to ruin someone/thing's day on some distant world.
Garry's Mod and Photoshop