Rifle rounds in The Long Dark are often rare. Use your ammunition wisely as it is not renewable. There are a finite number of rounds across the regions.
Rifle rounds are found either in a small, red and grey coloured cardboard box or alternatively if the player has a keen eye can be spotted singly in fishing huts and other buildings.
Hunting rabbits with the hunting rifle is not very effective, as the rabbits are small and fast, and they can be hunted with stones found on ground instead of ammunition. So you can save your ammunition for more difficult hunts.
Ammunition spawns randomly throughout the regions, however there are some areas that are more likely to have boxes of ammo.