[kdogg]

A spring no stiffer then the M120 or something equal. Could always get a tight bore too, but not needed.

Classic Army guns are designed to handle upto the Systema M120 or something equal to this spring, stock, out of the box with no other upgrades. An M120 will get you roughly 390~410 FPS depending the gun.

I got 419~423 FPS on a customers SAR M41 with a Systema M120 and tight bore. Took out the right bore, dropped down to about 405~409 or so. This was before the spring settled btw which can take upwards of 5,000+ rounds.

Going from a M120 to an M130 is like a whole new style of spring. The M130 is ROCK HARD stiff. The gears and piston in a CA wont like you using a M130 or higher too long. If you need that type of power, you are going to need better gears too.