I've worked on a couple of rifles in the past week or so. All ended up with Modify S--- springs.
Some had torque up gears...some were "mostly" stock...some were tweaked to the edge...some were in rough shape when they came in (i.e. opened the mechbox and broken bits of stuff came out). These are all chronied with hopup off.
KWA M4 + S100 = ~374fps w/0.20
KWA M4 + S90 = ~345fps w/0.20
G&G M4 + S90 = ~301fps w/0.20
G&G M4 + S100= ~335fps w/0.20
RS AK + S90 = ~301fps w/0.20
RS AK + S110 = ~385fps w/0.20
custom M4 + S110 = ~415fps w/0.20 (oops)
custom M4 (CQB) + S100 = ~360fps w/0.20 (oops)
KWA M4 + S90 = ~344fps w/0.20
KA FAL + S110 = ~390fps w/0.20 (sweet)
KA FAL Carbine + stock spring = ~350fps w/0.20 (supra sweet...it's mine
KA FAL Carbine + S110 = ~399fps w/0.20
CA M4 + M110 = ~420fps w/0.20 (!!!frack!!!)
CA M4 + M100 = ~387fps w/0.20
There's probably a couple of others somewhere in there...but I can't keep track of them all....
One of the best ones was a tiny AKS74U that I did up for a shooter...S120 = !!!485fps w/0.20!!!....S110 = !!!455fps!!! and finally S100 = 405fps w/0.20. How does a rifle with a tiny little barrel shoot WAY over what the spring "should" shoot? I'm not telling....LOL ('cause I don't know)
Things to note:
1. There is NO
"magic" 100% reliable formula for slapping in a spring and getting a certain velocity...there are only ranges (and the ranges are at a min +/- 20fps)
2. Buy the spring that you "think" will put you in that range...and then buy the next one down as well.
3. A chronograph is a MUST...shooting pop cans and just guessing is irresponsible and laughable
4. Stock gear ratios can pull M90/100/110/120 spring fine...they struggle a bit at M120 and make the motor work hard.
5. You can assemble a rifle, disassemble it and reassemble it...and your FPS can easily vary between the assemblies by as much as 5fps
6. Aim lower than the Maximum field limit
...300-330fps is plenty for indoors, 380-ish is plenty for outdoors. You're not really going to get a lot more range from another 10fps...if you're shooting hot at a limited FPS game you're an asshole cheat...if some players aren't calling their hits after you shoot them repeatedly, they're not going to change because your rifle is shooting 10fps faster.
7. I should spend more time playing than fixing stuff...
(magic formula: Modify bearing spring guide + Modify bearing piston head + Modify S90 = mid 340's fps...if you buy from Airsoftparts.ca....but don't tell anyone else...)