I have a dozen higer end springers and find the Diana 350 to not be a gun that you shoot often because its very difficult to shoot accurately, I have found beeman crow mag to be the most consitent, this gun is accurate but you have to hold right while resting on a benchrest, What spring can replace the stock spring to tone it down a bit. I have heavy tar on the spring.
12 fpe spring or do as I have stroked the gun and used an HW98 SPRING. Hand fit guide and top hat and no TAR.
YOU can find a spring that will lower the power or cut the spring u have by 2 coils.
Striking the 350 and clones change the shot cycling much. Less recoil and still power. JME
If power isn't a big hot button for you, and accuracy is, you might even be better-served by cutting 3-4 coils from the spring. In doing that, you should still be pretty well above the 12 FPE mark. If you do that and the power dips a little beyond what you were hoping for, you can always add a .875" OD spacer or two (sold by James Maccari/ ARH) and bring the power back up a little and still not have as much velocity and hold sensitivity as where you started. If you add one of those nylon spacers, make sure you add a steel washer in between the spring and the spacer, and polish the ends of the spring so the steel spring doesn't chew-up the spacer.
So, at this point the question arises, do you know how to properly cut, close, and grind the ends of an air gun mainspring? If you do it right there shouldn't be any problem with spring power and longevity.
check your stock screws too. My .177 and .22 350M stock screws(the ones on the fore end, right/left) have a tendency to back out after a few shots and cause accuracy issues. The screws need some kind of light threadlocker on them, like VibraTite.
