I put a 17 pound spring in my camp carbine and also have the recommended 21 pound spring for it. I need to shoot the 17 pound spring and see how it does. The 11 pound spring from the factory is reported to be too light and some get cracked stocks from recoil.
I think if I installed the 21 pound spring I could load light 9mm lead bullet loads that would not cycle the action and use it like a straight pull bolt action. Great for quiet small game hunting and just the sort of experiment I like to do.
If you find one for less than $500 pick it up. They are fun little rifles. I have had two of them. We shoot them from my buds front porch at a 3.5" pipe on the far side of his pond that was formally used to hold a feeder to feed catfish. Shooting off hand the record so far is 8 out of 10 shots being hits. I have shot it out to 200 yards and could keep my rounds on a mans chest at that range. But a hundred yards or less it should be a deadly little gun.