Hey, that nesting spring idea, it won't work.
Just thought I should let you know.
I don't actually own a Red Ryder, or any other Daisy BB gun like it, so I'm glad you posted that picture. I'm sure you already posted it, but at 23 pages and counting, I didn't feel like searching for it.
I do have several other spring-powered projectile launchers though, so I do have a little bit to go off.
Here's another idea... There are coil springs that are flat (find a picture of the S&W Shield recoil spring, you will understand what I am having difficulty explaining). What if you got an inch long (or so) spring like that and put it where your preload spacer is? It is flat so I'm hoping that will help the round spring from slipping off and binding. Might be easier than getting extra strong springs. Might just be a nightmare. Like I said, I have lots of ideas. Eventually, I'm bound to have a good one!