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Buy a 100% or M90 spring if you want to be sure you're going to be under 350fps. i.e. Modify S90.
A 110% or M100 spring will probably be ok...but you might be just over.
Don't worry too much about the lables...different manufacturers use different notations. Some are rated based on Meters/Second (multiply by 3.3 to get FPS). Others are rated on % of 290-300fps (which is the "TM stock speed"). So a M100 = 330fps...a 100%=290-300...a 110%=320fps...etc...
The numbers are going to be off depending on your specific setup...how complete the airseal is...other parts that "boost" the FPS a bit (i.e. bearing spring guides/piston heads...tight bore barrels...etc)
So if you want to hedge your bets...go with 1 power level down from the theoretical.
And...the ONLY way to actually know is to use a chronograph.
Hope that helps.
Airsoftparts.ca has all the parts....tell Jugglez you want a Modify S90+ spring...tight pack of coils go at the spring guide end.
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