[L473ncy]

It actually depends on brand. You're not going to get the same power from a SystemA M100 as you would from a Madbull M100 or a Deepfire M100, or a Prommy M100, or a Modify SP100. They're general guides only to be used to compare within the brand itself. That said an M90 should be good enough (and after breaking in the spring should be perfectly fine).

Generally though you can kind of guess what FPS range you'll be shooting at by installing X brand spring. For example with an M120 you'll usually get 380-410-ish FPS. M110 350-380-ish. etc.

Also airseal in the compression unit plays a part. If your piston leaks a lot of air then you're not going to get the full power from an M120 spring. If your compression is as perfect as it can be on the other hand you'll take advantage of all the power from the spring that possibly could be applied to the BB (accounting for inefficiencies and energy losses).

Note that ported cylinders also play a part in this. But that's a bit of a more advanced topic (truth be told I'm a bit lazy right now and don't want to type up a whole essay).