Diagnosing and repair of doubling in Parker SST:
1. Both hammers fall at the exact same time - one loud report. This usually indicates that the barrel selector has not fully seated in one position or the other. The selector may exhibit stiffness when changing barrels or may not remain fully in one position or the other. A properly operating selector should only require moderate effort to move and have a tactile response as the spring slide engages the detent. The barrel selector is secured to a spring slide with a brass drift pin inside the receiver. This pin can slide out of position with use and come into contact with the trigger causing the selector to remain out of position and drop both sears at once. This is quite easy to repair.
2. Hammers fall sequentially - zipper like report. This kind of doubling indicates a worn sear - recoil of first shot cause second sear to slip and drop the hammer. Usually occurs when selecting a particular barrel first. ex. Selecting right barrel first - no double; selecting left barrel first - frequent doubling occurs. Repair requires worn sear to be reground. Not a particularly simple repair.
Hope this helps.
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