im hoping to one day hunt, which is why i shoot / practice with 1/2" steel. i heard 1/2" steel is optimal for rabbits, which would be the largest game i would consider hunting. Also, i watched a video about someone talking about how tapering doesn't always guarantee higher speeds, but from what i hear from others that info is probably wrong. With the 1/2" steel i can only dent an empty soda can at about 40 feet away, instead of at least penetrating the cans first layer, which is what im hoping to do
If you want better performance, the tapers really are a small bit of magic. They can also be an unending rabbit hole that you can lose yourself in searching for the best one for your favorite ammo. But some of us enjoy a bit of that now and again.
Also, the modern elastics really are better. Although Henry's point is well made- rabbits were well within reach for surgical tubing propelled ammo (sometimes rocks) in days gone by. It's just a question of how much tech you want to put into your speed search.
Spending some time playing with one of the newer latex brands that have been well vetted here may be fun and worth your while (Simpleshot, Precise, GZK, Sumike, Snipersling, BSB, to name a few, and probably forget a few). Same for tapers. Also, there is a treasure trove of info on bands and tapers if you search this forum- how to make them, what ones do well with this ammo or that, etc..
For the penetration- I don't have a lot of experience hunting with a slingshot, at least not in this or the last decade. But remember that it is the impact rather than the penetration that gets the job done. Okay, some of each really, but the bigger the ball the more impact matters. I have compared small and big ammo going at the same velocity and found the small will penetrate a can when the large will not. However, when doing then energy calculation the bigger ball is packing several times more power. So my guess is you don't need to worry about penetration so much- look at the mass of your ammo and the speed you are shooting it at. Maybe feed those into a bullet energy calculator app and see if you meet or beat one of the recommended numbers for hunting- I think I have seen rec's ranging from 6-10 ft lbs of energy for humane hunting of game like rabbits. Without a lot of personal experience I'm guessing if you can transfer 10 ft lbs of energy to a can regularly with ammo going 180 fps or better you'll be good to go. But just my semi-informed WAG.
For energy, I don't think any will argue much over 1/2" steel. Although the 7/16" seems to be very popular for hunting being a nice mix of mass and speed that keeps a lot of the power with good mass while shooting a good bit faster and thus giving flatter trajectory / more reliable accuracy.
If you take it, it should be an enjoyable journey that will improve things a lot for you. And if you get there by testing a couple elastics and tapers along the way, you may enjoy the trip and gain some valuable info along the way. And if you mine this forum for info, you'll find it to be a great map for guiding and speeding your journey- so much good info and many helpful people with vast experience here.
Cheers, best of luck, and happy shooting