RecoilMaster, STI - Replacement Spring

This is a replacement for the main spring in the STI RecoilMaster guide rod system.  This spring does not require replacement like a standard recoil spring.  Rather you may want to replace it if your current RecoilMaster spring has been damaged/altered, or if the pistol with this part installed has been shot more than 30 - 50 thousand rounds.  Also, these springs cannot be swapped out to alter the felt recoil of the pistol.  They are matched to the type of pistol: length of gun and caliber.  A 9mm or 38 Super pistol uses a "light" spring, whereas a 40 S&W or 45 ACP pistol uses a "heavy" spring.