This is the MagFox replacement magazine spring for the S&W 15-22 25 round magazine.
Fresh springs help maintaining optimal pressure for reliable competition feeding if your magazines have seen some use.
Also fits the CMMG Gen 2 AR-15 .22 magazine, providing extra power than the factory spring to reliably operate last round bolt hold catches such as the ARCatch22.
Can also be trimmed down for use in high capacity extensions for the GSG 1911.