Mass Driver Hammer Kit for S&W K/L-Frame Revolvers

This Mass Driver Hammer Kit replaces the factory MIM hammer, sear, stirrup, and spring in S&W K/L-frame revolvers. The core design increases mass behind the hammer face, delivering a higher-energy strike at the firing pin while allowing a lighter hammer fall weight. This directly addresses inconsistent ignition in .22 rimfire cylinders and enables a reduced double-action trigger pull weight without sacrificing primer strike reliability in centerfire chambers. Single-action function is retained.

Platform Compatibility

• Smith & Wesson K-frame (Models 10, 13, 14, 15, 16, 17, 18, 19, 64, 65, 66, 67, 617, 648, 681, etc.)
• Smith & Wesson L-frame (Models 581, 586, 686, 619, 620, etc.)
• Does NOT fit N-frame or J-frame revolvers
• Compatible with both rimfire and centerfire cylinders on convertible models

Key Features

• Increased hammer mass for reliable primer ignition at reduced mainspring tension
• Replaces factory MIM hammer, sear, stirrup, and spring with precision-machined components
• Allows 2–3 lb reduction in double-action trigger pull weight (dependent on spring selection and gunsmith fitting)
• Maintains single-action sear engagement and release point
• Includes sear, stirrup, and mainspring – no additional parts required for installation
• Drop-in fit with minor fitting on K/L-frame revolvers; some frames may require sear or hammer notch adjustment

Install Difficulty: 3 out of 5

Intermediate armorer level. Requires sideplate removal, mainspring replacement, and hammer/sear pin installation. Tools needed: gunsmith screwdrivers (hollow-ground), brass or nylon punch, hammer block removal tool (optional), and a vise or bench block. Fitting may involve light stone work on sear engagement surfaces if timing or trigger overtravel requires correction. Not recommended for first-time revolver disassembly.

Frequently Asked Questions

Will this kit work in a Model 686 with a .357 Magnum cylinder?

Yes. The Mass Driver Hammer Kit is designed for both rimfire and centerfire K/L-frame revolvers. In .357 Magnum, the increased hammer mass allows reliable ignition with a lighter mainspring, reducing DA pull weight while maintaining primer strike energy.

Do I need to replace the factory trigger or trigger return spring?

No. This kit only replaces the hammer, sear, stirrup, and mainspring. The factory trigger and trigger return spring remain in place. If you wish to further reduce DA pull weight, a reduced-power trigger return spring can be added separately, but this is not required for the Mass Driver function.

Will this kit affect single-action pull weight or safety?

Single-action pull weight is not significantly reduced by this kit. The single-action sear engagement is retained, and the hammer fall weight is unchanged in SA mode. The factory single-action notch geometry is preserved, so no increase in sear slip or unintended discharge risk occurs when correctly installed.

Shipping & Warranty

Free shipping on orders over $75 within the continental US. This component carries a limited lifetime warranty against defects in materials and workmanship. Warranty does not cover damage from improper installation, modification, or use of non-specified ammunition. Install at your own risk – Performance Trigger Specialists recommends professional gunsmith fitting for revolvers.