Whilst at the summer cottage, we helped clear up the area, filling an entire skip with trash and unwanted stuff. One of those things we chucked was an ancient trampoline that's just been rusting away unused. Before it all went in the skip though, I liberated 6 springs which I'm now hoping to make some sort of arrow/bolt throwing device. Judging by how stiff they are, I'm thinking just the one spring will be more than enough power (I'm going to have to draw this thing and trust my building skills so I really don't want to overpower it and have it explode in my face). I just tried to see how much force I could get out of it and with fairly little stretch I got around 30kg/65lbs of force. I'll post a quick sketch of what I'm thinking of doing but it will be using rollers and with a bit of extra leverage I should be able to get higher numbers than that.

One thing I might need some tips for is the trigger mechanism. I need it a simple and strong as possible. Any advice? Any suggestions for bolts?

Very poor drawing but this is the basic idea (missing the trigger mechanism and much blockier than I'm hoping it will be). It would have two wheels on each side of the stock. The string stop is something else I need to have a think about.

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Tried again and got 35kg/around 75lbs. I'm thinking that if I can come up with a good way of loading it, I might be able to use two springs. I think that might need a winch though; I'm pretty weak :D

springs are a strange way to accelerate a projectile. they can in most cases be replaced by rubberbands.
If for any reason you feel there is too much kinetic energy wasted for the movement of the spring then you can put another pulley on the spring so you get double the movement/distance on the arrow for a given distance travelled by the spring....

or if you want a bigger ratio than 1:2 you can make a spring powered trebuchet/ballista and use 6 springs.

I'm mostly using them because I have them. I also like the idea of using springs just because they have that slight Mad Max vibe :D The wheels have arrived but I haven't made any progress because I've just not had the time. I should be getting smaller but even stronger springs that have come off of a swing seat so I might be using those instead which might lead to a redesign. Have to see what they're like first.

Rubber bands are better if you can get a steady supply, but if we’re talking Mad Max... You can make a spring any length you want from heavy-duty monofilament fishing line using this technique:
https://www.youtube.com/watch?v=1A2LUbJjDQ0

Once I was messing around with a handheld drill in the garage and made a couple of springs that were about 2 feet long each. I built a weird hybrid crossbow/slingshot contraption out of pvc pipe and launched a dowel with surprising power... the hole in the wall is still there to prove it!

With a crossbow type set up you could get some decent power.  A very strong spring with just a few inches of movement could make a powerful bow with the right cams. Might have to cannibalize an old compound for those. The cams themselves would have to be very light and strong as well. Sounds like a fun project.

I thought about cams but the way I see it, to have them properly set up, everything has to be very exact and, with the potential power something like this could have, the last thing I want is it blowing up in my face. As in literally blowing up in my face... With the design I have now (I should post the updated plan here soon) it's as simple as it can be, with the fewest moving parts, I'll be overbuilding it to ensure it's super strong and, if something does go wrong, I'll have the stock between me and the spring and it would (hopefully) explode down and away from me. There's a lot of different designs I could do but considering the space and tools I have, simple will be best.

https://www.youtube.com/watch?v=PZvrk-L6Gzg

Don't know if you saw it but it might give you some ideas

Sarosh wrote on Mar 20^th, 2020 at 7:52am:

https://www.youtube.com/watch?v=PZvrk-L6Gzg Don't know if you saw it but it might give you some ideas

Wow!

I did! I've been wanting to get on with this project but now I'm kind of wanting to avoid unnecessary shopping trips... I don't think mine is going to look quite as nice as that one but if I could get even half the power out of the springs I have, I'll be very happy.