When I was a kid I saw a rocket car at the local drag strip. No, not a jet car, (though I've seen those as well) a rocket car. It had three stainless cylinders. One large one, and two smaller ones. They lined it up against a Top fuel car (Funny Car).
The rocket car went by and I said wow, that was quick. I thought the funny car had broke, but when I looked back towards the starting line here it came. That rocket car set new track records for ET and MPH on that pass. I can't recall the figures but it just crazy fast.
A few years later a guy had a smaller version on a motorcycle. I read about it in some magazine. The problem was he couldn't hang onto it. These rocket vehicles seem to be limited by the human factor. I don't think they burn fuel but for about half the track, but it is one heck of a shove!
Whether this sort of thing could be minaturized down to something you could use in a rifle, I don't know. But if it could it would be interesting.
Here's a record set back in 1977, by a girl no less.
"1977 was the year... Kitty O'Neil was the driver...Ky Michaelson was the mastermind...Space Age Racing was the team... a record in the quarter-mile was the goal. Not only did the team reach their goal, they smashed it! Kitty ran the quarter-mile in 3.22 seconds at a speed of 412 mph."
http://www.the-rocketman.com/space-age-racing.html