Thanks that is a whole lot more info than I expected.
I don't wand to build a dmr just want to increase the fps as it's currently around the 230 fps,
But and this was my initial thinking "change out the stock piston/head, oh will iv got the gearbox apart I'll install a bigger spring not too much bigger than stock, umm will the gears cope I'll change them just in case. No idea what I need I'll ask the guys on the website"
I did just that and your advice has set me stright change one thing at a time.