Originally Posted by SNK
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 ).
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.