I was reading this question on the Worldbuilding SE about stray bullets eventually hitting other things, like a space station or another ship, somewhere else in the same solar system. The consensus is that while the odds are extremely small (because Space is Big), it's still plausible if the plot demands it. Until it hits something, or burns up in some atmosphere, a bullet fired in the vacuum of space will just keep going.
But space isn't really a vacuum. There's lots of particles floating around in a solar system, and even the remotest corner of interstellar space still has a few hydrogen atoms flying around. My question is, would these particles slow down a projectile and eventually stop its forward motion? Would that happen within the same solar system, or would the process take too long?