I replaced the spring in the MP44 with a 1J Systema. It was the only one I have still in it's packet so I was confident of the contents - I have loads of other springs but no idea of their power 'cos they are just rolling around in the box 
Gawd knows what spring was in the gun originally to get 520fps but it was shorter than the 1J Systema (and all the others I have) which meant when the 1J was inserted in the box there was a degree of bend in the spring, this wasn't the case with the original.
Anyhoo, the 1J is giving me 275fps. I might use the MP44 at UCAP so I'll keep it as it is for the moment but to get it up to a more useful 350+ fps later on, what spring do I need to get, given 1J gives 275fps??
Systema springs are 'orrible - especially the 1J ones. Use PDI. Depending on how good the component-to-component seals are (i.e. piston O ring to cylinder wall, air nozzle to cylinder head etc etc) which lets be honest, doesnt tend to be perfect in ACM guns, stick a PDI 140% in and you should get between 350 and 375.
Reason that the systema sping is so long is that they use thinner cross section wire to make them from (which is why the occasionally snap!) and just give the spring more coils. PDI use thicker wire, and less coils, so its shorter and easier to deal with. Oh yeah, whilst you're at it, get a King Arms or Systema bearing spring guide - it makes it MUCH easier to reassemble a V2 box.
When we were a Kingdom it was run by a King
When we were an Empire it was run by an Empress
Now we're a country we're run by a..........
Ah, thanks for that, very clear advice!
On a related note. What sort of price would I be looking at to up the range on my Thompson? (ie take it up to 350fps)
It does about 280.....it's old and knackered.
As I've no idea whatsoever about these sorts of things, I don't know what I would need to do it.
All depends what condition its like on the inside. I'd have to look inside the gearbox to give a proper appraisal...
When we were a Kingdom it was run by a King
When we were an Empire it was run by an Empress
Now we're a country we're run by a..........
