Honestly I forgot what my original inspiration for my idea was. I know part of it was the Slingshot crossbow made by Joerg. Anyway looking at that speargun rubber I also wondered if some form of mechanical advantage could be used to make that kind of poundage practical. The thread here https://www.theslingshotforum.com/f18/speed-testing-different-pulley-systems-34695/ seems to confirm that pulleys can be used to be effective. Central to my idea is a pair of special pulleys. This pulley is actually two pulleys mounted together along a shared axis. The large pulley has the bow string wound around it and attached to it. The small pulley has a string attached to it that leads to some very heavy rubber like speargun rubber. Since the two pulleys are conjoined they turn together. The difference in size between the pulleys would create a mechanical advantage ratio depending on how much bigger the large pulley is compared to the small one. Though I think 2 to 1 is high as you can go without it being too big. its kind of hard to me to describe my idea in words so I am posting a picture so you can better see what I'm talking about. Also Idk if such a pulley already exists or would actually work. If any of you know better please let me know.