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

If you’ve been fighting light strikes with your .22 rimfire revolver, or you’re trying to lighten the double-action pull on your centerfire S&W K or L-frame without losing reliable ignition, you’ve found the fix. The Mass Driver Hammer Kit replaces the factory MIM hammer, sear, stirrup, and spring with a heavier, precision-machined hammer assembly. The extra mass behind the hammer face means you can run a lighter mainspring—and get a lighter trigger pull—while still getting solid primer hits every time. This kit is designed to work in both rimfire and centerfire revolvers, so you don’t have to choose between a smooth trigger and reliable ignition.

Compatibility

• Smith & Wesson K-Frame revolvers (Models 10, 13, 14, 15, 16, 17, 18, 19, 48, 53, 64, 65, 66, 67, 617, 648, and similar)
• Smith & Wesson L-Frame revolvers (Models 581, 586, 686, 619, 620, and similar)
• Fits both rimfire (Model 17, 18, 617, 648) and centerfire variants
• Does NOT fit N-frame or J-frame revolvers
• Does NOT fit pre-Model numbered guns without fitting by a gunsmith

Key Features

• Heavier hammer mass – Adds weight behind the firing pin for more consistent primer strikes, even with reduced mainspring tension
• Drop-in replacement parts – Kit includes hammer, sear, stirrup, and spring to replace factory MIM components
• Lighter double-action pull – Allows you to reduce hammer fall weight without sacrificing ignition reliability
• Maintains single-action function – Your revolver still cocks and fires in single-action mode just like factory
• Rimfire-optimized design – Originally developed for .22 LR, but works great with centerfire loads too

Install Difficulty: 3 out of 5

This is not a drop-in trigger shoe. You’ll need to detail strip the revolver frame to swap the hammer and related parts. Expect to need a good set of gunsmith screwdrivers (hollow-ground), a slave pin or two, and a bench block. A mainspring strain screw fitting tool is also helpful. If you’ve replaced a revolver hammer before, you can do this in about 45 minutes. If you haven’t, we recommend watching our install video on the product page—or sending it to a smith. We offer installation service if you’d like to add it at checkout.

Frequently Asked Questions

Will this work in my Model 686-6?

Yes, the Mass Driver Hammer Kit fits all L-frame revolvers including the 686-6, 686-5, and earlier dash numbers. It also fits the Model 586.

Do I need to change the rebound spring too?

Not required, but many shooters find that pairing this kit with a reduced-power rebound spring (like our 13-lb spring) gives an even smoother double-action pull. The kit itself only replaces the hammer, sear, stirrup, and mainspring.

Will this fix light strikes on my .22 LR Model 617?

Yes—that’s exactly what this kit was designed to solve. The heavier hammer mass gives more consistent primer hits with rimfire ammunition, even when you lighten the mainspring. Many customers report zero light strikes after installation.

Shipping & Warranty: We ship same business day on orders placed before 2 PM Eastern. This kit is covered by the manufacturer’s lifetime warranty against defects in materials and workmanship. If