{"product_id":"sw-k-frame-mp-1st-model-4-screw-main-spring-38-colt-mpc13","title":"S\u0026W K Frame M\u0026P 1st Model 4-Screw Main Spring .38 Colt – Part# MPC13","description":"\u003ch2\u003eSmith \u0026amp; Wesson K Frame Military \u0026amp; Police 1st Model Main Spring – 4-Screw, Old Style\u003c\/h2\u003e\n\n\u003cp\u003eA rare factory-original main spring (mainspring) for the earliest production K Frame revolvers. This part was sourced from a Military \u0026amp; Police 1st Model revolver (Serial# 3312) chambered in .38 Colt — one of the earliest S\u0026amp;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.\u003c\/p\u003e\n\n\u003cul\u003e\n  \u003cli\u003e\n\u003cstrong\u003ePart Number:\u003c\/strong\u003e MPC13\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eFits:\u003c\/strong\u003e Smith \u0026amp; Wesson K Frame Military \u0026amp; Police 1st Model (4-Screw)\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eCaliber:\u003c\/strong\u003e .38 Colt\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003ePart Type:\u003c\/strong\u003e Main Spring (Mainspring \/ Hammer Spring)\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eEra:\u003c\/strong\u003e 4-Screw, Old Style\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eFinish:\u003c\/strong\u003e Blue Steel\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eProvenance:\u003c\/strong\u003e Sourced from Serial# 3312\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eCondition:\u003c\/strong\u003e Used (factory original)\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eInstallation:\u003c\/strong\u003e Gunsmith fitting recommended\u003c\/li\u003e\n\u003c\/ul\u003e\n\n\u003ch3\u003eMain Spring \u0026amp; Trigger Pull\u003c\/h3\u003e\n\u003cp\u003eThe 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.\u003c\/p\u003e\n\n\u003ch3\u003eFactory Replacement Quality\u003c\/h3\u003e\n\u003cp\u003eGenuine Smith \u0026amp; 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.\u003c\/p\u003e\n\n\u003ch3\u003eAbout the M\u0026amp;P 1st Model\u003c\/h3\u003e\n\u003cp\u003eThe Military \u0026amp; Police 1st Model (introduced circa 1899) was one of the first S\u0026amp;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.\u003c\/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eQuestions?\u003c\/strong\u003e Call or email us — we're happy to help you get the right part the first time.\u003c\/p\u003e","brand":"Smith \u0026 Wesson","offers":[{"title":"Default Title","offer_id":40859074396218,"sku":null,"price":16.99,"currency_code":"USD","in_stock":true}],"thumbnail_url":"\/\/cdn.shopify.com\/s\/files\/1\/1423\/2020\/files\/s-l960_27_f7f9adfb-200f-41a0-9fe8-8bb4ea6dfb90.webp?v=1745860382","url":"https:\/\/usagunsandgear.com\/products\/sw-k-frame-mp-1st-model-4-screw-main-spring-38-colt-mpc13","provider":"USA Guns And Gear","version":"1.0","type":"link"}