I have always tried to keep a recoil spring that was 10 1/4", but no shorter than 10 1/8". I had a 9 7/8" spring that worked OK on Aguilla (underpowered) ammo, but would not feed from the top of a 30 rounder.. In an AR15, the carbines and M4s are vary dependent on correct recoil springs. I generally get a USGI M4 spring NIW when I can find one for CAR-15 builds.