S&W K Frame M&P 1st Model 4-Screw Main Spring .38 Colt – Part# MPC13
$16.99
Skip to product information
S&W K Frame M&P 1st Model 4-Screw Main Spring .38 Colt – Part# MPC13
$16.99
Smith & Wesson K Frame Military & Police 1st Model Main Spring – 4-Screw, Old Style
A rare factory-original main spring (mainspring) for the earliest production K Frame revolvers. This part was sourced from a Military & Police 1st Model revolver (Serial# 3312) chambered in .38 Colt — one of the earliest S&W hand ejector revolvers ever produced. The main spring powers the hammer, providing the force needed to strike the firing pin with sufficient energy to reliably ignite the primer — a weak or broken mainspring causes light strikes, misfires, or complete failure to fire. Correct-era mainsprings for these early 4-screw revolvers differ in geometry from later production and are extremely difficult to find.
- Part Number: MPC13
- Fits: Smith & Wesson K Frame Military & Police 1st Model (4-Screw)
- Caliber: .38 Colt
- Part Type: Main Spring (Mainspring / Hammer Spring)
- Era: 4-Screw, Old Style
- Finish: Blue Steel
- Provenance: Sourced from Serial# 3312
- Condition: Used (factory original)
- Installation: Gunsmith fitting recommended
Main Spring & Trigger Pull
The main spring works in conjunction with the strain screw to set mainspring tension and directly affects trigger pull weight in both single and double action. Replacing a worn or fatigued spring with a factory-spec part restores the original trigger pull weight and ensures reliable primer ignition. On these early revolvers, mainspring geometry differs from later K Frame production — always verify fit before installation.
Factory Replacement Quality
Genuine Smith & Wesson factory part from the correct production era — same spring rate, same dimensions, same reliable hammer energy. Correct-era parts for these early revolvers are increasingly scarce.
About the M&P 1st Model
The Military & Police 1st Model (introduced circa 1899) was one of the first S&W swing-out cylinder revolvers and the direct ancestor of the legendary Model 10. Early production revolvers like Serial# 3312 predate the model number system by decades — if you're unsure whether this part fits your specific revolver, contact us with your serial number and we'll help confirm compatibility.
Questions? Call or email us — we're happy to help you get the right part the first time.