So you're talking about building a genuinely functional weapon, which starts to get off-topic for this board. And if you hurt anyone or anything using any of this advise, it's your own dumb fault.
anyway, it's turns out that crossbow bolts are surprisingly easy to make presuming forests still exist. The only tools you really need are an axe, a froe, a die (a hunk of steel with a hole punched in it), and a knife. You chop down a tree; chop it into bolt-lengthed logs, use the froe to split the log into 3/8'' square sticks. Hammer each stick through the die to turn them into dowels. You sharpen the dowels to a point with a knife, and you bake the point over a fire just short of charring it to harden it. If desired, you can nap either flint or scrap glass to make a harder sharper tip. You can either attach feathers to the end using home-made hide glue, or you can cut a notch and slide in a pieces of thin plastic or soda can aluminum to stabilize it. With a bit of practice, you can make a hundred bolts in an hour.
Things like marbles and ball bearings are not going to be able to penetrate enough under human power to kill medium game (deer). They're only good for small game like rabbits and squirrels. For that, you should just build yourself a quality slingshot and carry it separately.
I expect you would have less frustrations using a compression or extension spring as opposed to a coil torsion spring. It's tough to find coil springs that can store that much energy, and each shot would put a lot of strain on the rope/pulley mechanism.