You wont get this on your first playthrough. You'll have to go back and do a 2nd playthrough and/or do the Challenges to rack up the bullet count. I suggest you constantly hold down the button as you play to speed this process along. One playthrough doing this will net you around 50-70k bullets fired depending on deaths, etc!
The best way to get this achievement is to do the rubber band trick. You'll get about 7-10k worth of bullets in about 1 hour or less.
If you want to see how many shots you've fired, just head to the Stats computer which is in the same room as Gerda to the right.