To give a little background on the topic of pellet weight as it pertains to spring- or gas-piston rifles, the most credible explanation for why too-light or too-heavy pellets are more stressful is made in the Cardews' book "The Airgun: From Trigger to Target".
The progression is something like this:
1. A "too light" pellet starts moving too early which leaves an insufficient air cushion in front of the piston to help decelerate it, thus it slams into the end of the compression tube. This abrupt impact is stressful to both the seal and the spring.
2. A "just right" pellet allows the piston to come to a well-behaved stop at the end of the compression tube.
3. A "too heavy" pellet stays in place too long and the pressure builds much higher, causing the piston to bounce back off the cushion of air in front of it, forcing the front portion of the spring backward while the momentum of the back portion of the spring is still propelling it forward. That can cause coils somewhere in the middle to collide with each other. Meanwhile the elevated pressure simultaneously creates a greater temperature spike which promotes detonation of any trace hydrocarbons in the compression chamber, forcing an even more violent reversal of the spring and potentially cooking the seal.
I don't claim to know if the above is 100% complete and accurate but there does appear to be a more violent recoil when using super light or super heavy pellets in most springers I've shot. And it stands to reason that given the relatively long dwell time a pellet spends in the barrel, that these "extra vibrations" are not good for accuracy.
The optimum weight for a given rifle will depend on its powerplant. Reading a bit on Pyramyd Air this evening on your model rifle, a reviewer said they were getting 690fps with 15.9gr JSBs. That works out to just shy of 17fpe, and suggests to me your rifle will probably be happy with pellets no heavier than that, and perhaps those closer to 14gr will hold more potential (e.g. JSB Jumbo RS 13.4gr, JSB Jumbo Express 14.3gr, H&N Field Target Trophy 14.6gr).
However if you want to work it out experimentally and get a sense of the optimal weight range for yours, what you can do is chronograph several pellets that span a range of weights. For example in .22cal, that may be 13gr up to 18gr. What you'll typically find is that very light and very heavy pellets produce less energy (FPE) than those in the middle. And what people often find is that their magic pellet is one that is at or very near the max energy. Logically it is also an indication of where the power plant is working most efficiently, and less prone to piston bounce and vibration. More of the spring's stored energy is going into propelling the pellet.
