Spring Rating (M-Number)

M-numbers aren't a standardised physical unit — they're a manufacturer convention going back to Systema's original spring line — but higher numbers reliably mean more force and, all else equal, higher FPS and joule output. Fields commonly express their power limit as a maximum recommended spring alongside the joule cap.

Fitting a spring heavier than a gearbox's other parts can handle risks stripping piston teeth or cracking a gearbox shell, so upgrades usually pair a spring change with reinforced gears and a piston rated for the new load. Spring choice also directly affects rate of fire: heavier springs slow the cycle down.

Field power limits are almost always quoted as a joule or FPS ceiling rather than a spring number directly, since the same M-rated spring can still produce different output depending on the rest of a gun's internals.