Nice link to that pyramid air HW50 @TonyL , that totally explains about those PG4 kits now, I was told they were adjustable but never understood how, an adjustable corkscrew type top hat to be able to fine tune the final output

Due to suffering with a bit of the tennis elbow if I constantly use break barrels , I've always been in favour of the softest springs I could get away with. I reckon a 10 year old could break my .177 TX and that has a slightly shortened No.10 spring in it.

I can't say I've ever tried a AA TX MK3 spring in anything else , after reading figures somewhere about how much softer they were than a No.10 Titan, with the TX having a long stroke , nearly 30mm more than a 99s , I just never tried one in a 99s, when I got my used TX200 , it had all the usual metal mass still inside , large metal top hat and spring guide , 3 metal power washers , nothing fitted very well at all , loose top hat and very loose spring guide , hence quite a bit too much twang , with all that metal work inside it was only making just over 10.4 with soft Exacts , the cycle was so lazy it didn't really want to shoot harder FTT at all , it was as if the FTT pellet really stuck in the barrel and only released at the very last moment, then it was only low 9's . With the TX having a huge swept volume compared to most other guns , they can easily make power with very soft internals, not everbody else's choice though.

Personally I've always tried as hard as I could to get my springs spot on with no extra washers at all, let the spring have as much freedom as possible to keep it as soft as possible rather than put too soft a spring in and have to give it power washer based preload . I'm not a power hungry merchant, when you shoot a 11 fpe+ .22 next to a 10.5 in .177 , theres a distinct difference in speed of around 200 fps between the 2 guns , you can certainly hear it at close range

So I bypassed getting a new Mk3 spring and got a Titan No.10 with good fitting spring guides, it works well even if a shortstroke kit is fitted in the TX, still makes power easily.

With the 99s only having just over a 70mm stroke and a smaller swept volume , I didn't even try a MK3 spring in it, If a Full Titan No.10 might not make the figures in a .177 version , chances are a mk3 spring won't come anywhere near it by a long way, might be interesting with a new Mk3 spring in a .22 version 99s though. I don't have one to try though so can't help on that front