As you progress through the game, you will unlock better scopes.
Successfully complete the game to unlock x-ray vision. Press X after you zoom in with the scope.
Start a career and complete any level's two seasons to unlock the .44 mag.
Successfully complete the game on the easy difficulty setting to unlock Johanna, the sniper character.
Use the scent cover when sneaking up on any deer.
Get a deer feeder and a tree stand. Walk to an area with a lot of trees. Set up your tree stand and deer feeder. Get up in the tree stand, use the scent cover, then use your buck call. It may take several minutes to attract a deer. For best results, use your rifles.
When in a tree stand and you want to get out, look down at the ground first. Then, get out.