I have a 0.22" HW77 rifle rather than karbine that I bought new in 1987.
After about a year I fitted a Maximiser kit that included an Ox spring, guides, piston sleeve and assortment of lubricants.
The kit guaranteed a smoother, more consistant shot cycle whilst keeping the gun within the limit. I never checked the muzzle velocity as nobody bothered in those days!
Twenty-five years later I bought a Combro and finally checked it. Needless to say it was running hot!
I dismantled it expecting to find damage to the internals after all of the negative reports I had read about the Ox springs. However, I could find no problems and the rifle had always shot well. Maybe, much of the bad reports were based on some people jamming the biggest possible spring into the gun in a quest for power. Mine had far less preload than the original Weihrauch spring.
I decided that I would try a modern tuning kit to see how things had moved on during those years.
I bought an appropriate 25mm V Mach spring and guides, piston sleeve and seal etc and reassembled the rifle. Fortunately, I was now equipped with a chrono, because the muzzle energy kept climbing over the limit as the seal bedded in. I systematically removed all of the preload washers and still the gun climbed over the limit. Finally I collapsed the end coil of the spring which reduced the power to an acceptable level. I have since fine tuned the output by removing material from the thickness of the top hat and fitting a single shim washer, which I am able to change for summer and winter temperatures.
I understand enough now to appreciate that being a long barrelled 0.22" means that it makes power more easily that a 0.177" karbine and much of the problem emanates from this.
I like the V Mach spring because it is short and allows the gun to be dismantled very easily. However, I could probably have done much the same process with the Ox spring that I had been using and saved myself the cost of the kit.
It is also being suggested that the shorter, thicker springs produce a snappier shot cycle and I may yet go back to the original Weihrauch spring that I removed all those years ago!