A magnum springer will probably be all but useless, you need something easy to shoot and that has average to above average accuracy. I would say your gun has to be at least capable of 1/2 inch accuracy it the field target position or off shooting sticks at 30 yards. Now remember I said capable, for most 1/2 inch accuracy at 30 yards is much harder to achieve than most would have you beilieve. CO2 is also out because of it's temperature sensitivity.
Kliff just look at mid- range to low power springers, 12ftlbs is plenty to knock down 55 yard kill zones. Beginners often think a full 20 FPE is the answer, it is nice to have but usually only PCP's can take advantage of that kind of power. A 20FPE springer would be too harsh to shoot well, not that it cant be done because it probably can but no one I've heard of has shot really well with one.
My TX is considered powerful to most spring gun shooters, it shoots about 15.5 FPE, that is 910 FPS with a 8.4 grain JSB. All of the the sprngers i know of at our club are less powerful than mine and they have done well nationally.
Now on the other hand my USFT shoots at 19 FPE about 920 FPS with a 10.5 grain Crosman Heavy, it shoots just as flat as my lower powered TX.
Also be mindful that alot of the PCP's shoot harder than the 20 FPE limit, so make sure you pick one with a power level that fits into the rules.
I fyou want a cheap scope that would work, I'd have to say a leapers 8-32 sidewheel would be the way to go, or a 6-24 bushnell banner mil-dot.
