Stephen Hunter has never been a sniper, but he's written some convincing sniper novels. CJ Box, who happens to be my current favorite mystery/suspense/thriller writer, writes some great crime fiction and he's never been a cop.
They say we should write what we know. Well, you know college life, so write a thriller in a college setting and fill in the blanks with research. If you need help with police procedural issues, gun information, or self-defense techniques for fight scenes, check out my blog for writers called
Righting Crime Fiction. It's in the early stages, but I've covered a lot about guns so far and am working on interviews at the moment.
Best of luck to you.