A 'can' has it's place but it makes a poor crutch when you are lacking in almost everything else. It does NOT make you Superman. It's also not absolutely indespensible. In other words, they are neat but you can do without it and they sometimes present problems, like if you get caught with the bitch.
A decent slingshot is a serious small game getter, like for quail. They are silent, in expensive, weigh little, can fit into a pocket, and ammo is in endless natural supply. I recommend the folding type that uses surgical tubing.
You need a decent knife - period.
You either get a big knife or you get a small knife, like a good skinner, and a hatchet. I'd recommend all three.
Trying to do things like using a straight razor to skin a deer or a range cow simply won't make the grade.
You might want to take a look at this:
http://www.knivesofalaska.com/
US Air Force 21st Special Tactics Squadron during field tests of gear in Montana called this set, "The most versatile, all-purpose field knife we have ever tested."
It's Knives of Alaska's 'Brown Bear' combo with beefed up military sheath designed for attachment to packs, web gear, and for airborne ops.