springs?
 

What is the difference between springs? m100 ect.

their strength. bigger the number the more power behind it.

the higher the number the higher the fps?

Yup.

M100 IIRC is supposed to be the same as a TM spring. M90 90%, M120 120% etc.

Although there are companies who have their own system. Best bet is to check out the spring chart somewhere on this forum (it's also been posted around on redwolf and Arnies IIRC).

look no farther: ASC Spring Chart

M means meters per second for that spring.

m100 means 100 meters per second = around 328 FPS.

Don't confuse that with springs that say 170% Those mean 170% of stock (stock around 280 FPS, 170% of that is 476 FPS ).

SNK is right, M-ratings mean meters per second and %-ratings mean % of stock 280 FPS speeds.

Although some companies M-ratings differ from other companies, so it's sometimes kind of hand-wavy. That's where Illusion's spring chart comes in handy.

What does the S in modify's "s120" or "s110" springs mean?

It stands for "SWEEEEEEEEEET".

But seriously, those letters are arbitrary, just to identify the product from the millions of other "M120" or "150%" on the market.

Likewise, Prometheus chooses to use "MS100SP" as their naming convention.

It's probably also worth mentioning that there's going to be some place in the final velocity, depending on barrel length, type, cylinder porting, how good a seal you're getting with your piston, etc.

I've never had a gun chrony "exactly" (or even really close to ~) the spring's rating, there's always maybe +/- 20 fps or so difference.