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

This Mass Driver Hammer Kit replaces factory MIM hammer, sear, stirrup, and spring assemblies in S&W K/L-frame revolvers. The design increases mass behind the hammer face to improve primer ignition — originally developed for .22 rimfire reliability, but equally effective in centerfire guns. The added mass allows a lighter double-action trigger pull weight without sacrificing ignition consistency. Single-action function is retained.

Compatibility

• S&W K-Frame revolvers (Models 10, 13, 14, 15, 17, 18, 19, 64, 65, 66, 67, 617, 648, etc.)
• S&W L-Frame revolvers (Models 581, 586, 681, 686, 686+, 619, 620, etc.)
• Not compatible with J-Frame or N-Frame revolvers
• Not compatible with pre-Model number (pre-1957) or older five-screw frames

Key Features

• Increased hammer mass for improved ignition consistency
• Allows lighter DA trigger pull weight vs. factory hammer
• Replaces factory MIM hammer, sear, stirrup, and spring
• Maintains factory single-action function
• Works with both rimfire (.22 LR) and centerfire (38 Special, .357 Mag, etc.) loads
• Includes hammer, sear, stirrup, and mainspring

Install Difficulty

3 out of 5. Requires removing sideplate, trigger assembly, and hammer block. You will need a gunsmithing screwdriver set, pin punches (1/16″ and 3/32″), and a brass or nylon hammer. Knowledge of S&W revolver lockwork is recommended. Expect 45–90 minutes if experienced; consider a gunsmith if unfamiliar with S&W trigger assemblies.

FAQ

Will this work in my 686-4?

Yes. The Mass Driver Hammer Kit is compatible with all K-Frame and L-Frame S&W revolvers with standard hammer design. Post-1990 production (dash-4 and later) may require minor fitting of the stirrup pin. Test fit before final assembly.

Do I need to replace the trigger or rebound spring too?

Not required. This kit replaces only the hammer, sear, stirrup, and mainspring. If you want a lighter DA pull, consider adding a reduced-power rebound spring (12–14 lb) and a lighter trigger return spring. Those are sold separately.

Will this fit a Model 19-5?

Yes. The dash-5 and earlier K-Frame models use the same hammer and sear geometry as the -6 and later. No frame modifications are needed. Ensure your revolver is unloaded and cylinder is open before installation.

Ships within 1 business day. All Apex Tactical components carry a limited lifetime warranty against manufacturing defects. Returns accepted within 30 days of purchase if uninstalled and in original packaging.