Just thought I'd share some thoughts, experience and observations on "Tuning Kits". I know that even the description "Tuning" can generate some controversy, but bear with me. Just for clarity I have no commercial interest in any of the suppliers - I am just an air gunning enthusiast who has bought and fitted their products.
And so to the rifles, reasons for fitting a kit, and observations of the kit and results:
HW97k .177 - had a standard HW spring and very annoying twang. I fitted a VMach kit supplied by Steve Pope. Wow, what a transformation, vastly improved smooth cocking cycle, very accurate to shoot and nice dull "thud" rather than the original twang - very pleased with the result, still the most accurate spring rifle I own. The kit was of high quality and was straightforward to fit. Steve Pope did answer a question I had and was helpful. Cons: this is the most expensive kit I have fitted, and as I recall there was a leadtime from order as I recall (ordered direct from Steve not via BAR) - fitted circa 6yrs ago. Some people say the VMach action is a bit harsh, I'd be inclined to mildly agree but in my experience it is excellent in terms of accuracy - the proof is in the groupings. I have also shot my rifle against a Sandwell Field Sports stage 1 tuned HW97k (with me shooting both rifles) - and I preferred mine.
AA TX200.177 mkiii - this grouped nicely as standard but had an annoying twang. I fitted a Welsh Willy kit but it generated a minimum power output of 13.5 ftlb. I ended up having to modify the spring twice - now running at 11.3ftlb. Will was helpful and did offer to replace it but in the end due to time and postage I opted to mod it myself. It was not the simple just "drop in" product I thought I was buying but the components were generally of good quality (clearly the spring length/power was wrong though). After fitting the kit the twang was gone and the rifle groups nicely. Nice smooth cocking action and it felt even lighter to cock.
AATX200.177mkiii - Tinbum tuning - I bought this kit just out of interest thinking that I'd like to see how it performed with the original TX spring (I just bought the base package - top hat and spring guide). I have not yet fitted this into the gun but my first observations are that the quality and precision of the components are poorer that either the VMach or Welsh Willy kit. All components had rough edges and required smoothing with some very fine wet and dry paper. The spring guide is not an identical match to the original AA form, and it comes in 2 pieces rather than a single component - I can see no practical advantage for this and likely disadvantages due to unwanted vibration and wear. Nick was helpful when approached and his assembly videos are quite good. This was the least costly of the kits I have tried and had the advantage of being able to reuse the original AA spring - the guide tube fitted snugly. However, in my opinion there is quite a bit of room for improvement in quality of the design/ materials/ manufacturing of this kit.
AA ProSport .177 - Tinbum Tuning - I installed this for a friend. The kit is the same as the TX200mkiii. The 2 piece spring guide did not fit together very tightly and was wobbly (and was not identical in form to the original) . The pieces for the top hat did not allow dimensional matching of the original AA top hat, and there weren't enough power washers provided. All components had rough edges and needed sanding back with fine wet and dry paper for a smooth finish. Once fitted the cocking cycle was smooth enough. Accuracy has not been tested yet - still being shot in. This was the least costly of the kits I have tried and had the advantage of being able to reuse the original AA spring - the guide tube fitted snugly. However, in my opinion there is even more room for improvement in quality of the design/ materials/ manufacturing of this kit.
As a final note, for those who are thinking of such modifications then I would advise you to first source a chronograph and secondly to test your rifle for power with a variety of pellets (as an illustration I found 1 ftlb variation in power between RWS Superdome and JSB exact).
Hope this helps. Happy shooting.