The CMMG MK9 Firing Pin Spring 90BA419 (UPC 810046231340) is a small but critical replacement part designed to keep your CMMG Mk9 9mm AR platform or compatible MK9-based builds running reliably. Engineered by CMMG with precision and durability in mind, this firing pin spring helps ensure consistent firing pin engagement, dependable primer ignition, and long-term reliability in both pistol- and rifle-style Mk9 systems.
Whether you’re maintaining a dedicated 9mm AR-9, a CMMG Mk9 rifle, or any compatible MK9 setup, this factory-spec replacement spring is a must-have component for your spare-parts kit.
Purpose-Built for CMMG MK9 & 9mm AR Platforms
The CMMG MK9 Firing Pin Spring (90BA419) is specifically designed for CMMG’s Mk9 9mm platform, used in 9mm AR rifles and carbines, and is also listed for use with Kahr Arms MK9-type setups in some catalogs.
Platform: CMMG Mk9 / AR-9 9mm systems
Application: Firing pin spring for bolt/bolt carrier assemblies in 9mm AR-style builds
Function: Maintains proper tension on the firing pin to ensure reliable primer strikes and safe operation
If your AR-9 has started to show signs of light primer strikes, inconsistent ignition, or you’re rebuilding a Mk9 bolt, this OEM-spec spring is the correct part to restore factory performance.
Durable Steel Construction for Long Service Life
CMMG builds this MK9 firing pin spring from high-quality steel, chosen for its resilience under repeated compression cycles and rapid firing.
Key durability benefits:
Steel construction resists fatigue, set, and deformation over time
Designed to withstand the repeated impacts and cycling forces of blowback-operated 9mm AR systems
Helps maintain consistent firing pin energy shot after shot
This isn’t a generic spring—it’s a purpose-built OEM replacement designed to work with CMMG’s Mk9 firing pin and bolt geometry.
Factory-Spec Replacement – Simple Drop-In Installation
The CMMG 90BA419 MK9 Firing Pin Spring is intended as a direct replacement part:
Drop-in fit for CMMG Mk9 9mm firing pin assemblies—no modification required when used in the correct platform
Ideal for routine maintenance, bolt refurbishing, or troubleshooting light strike issues
Sized and tensioned to match factory Mk9 specifications, preserving the balance between reliable ignition and safe operation
For best results, install it along with a clean, properly fitted CMMG MK9 firing pin and ensure your bolt, channel, and components are free of carbon and debris.
Maintain Reliability & Performance in Your 9mm AR
Over time, any firing pin spring can weaken, especially in high-volume training or competition guns. Replacing it with a fresh CMMG MK9 firing pin spring helps:
Restore consistent primer ignition
Reduce light-strike malfunctions
Keep your AR-9 or MK9-based firearm running at peak efficiency
Retailers repeatedly describe this part as a “high-quality replacement” and an “essential component for maintaining reliability and functionality” in the MK9 system—clear recognition of the role this spring plays in overall performance.
If you rely on your 9mm AR for competition, training, or home defense, keeping a few of these springs in your parts bin is inexpensive insurance against downtime.
Compact, Affordable, and Easy to Stock as a Spare
Because it’s a small, lightweight component, the CMMG MK9 Firing Pin Spring is:
Very affordable, often priced around just a couple of dollars at major retailers
Simple to store as part of a field repair kit or bench spare-parts kit
Quick to replace during cleaning or bolt maintenance
Many gunsmithing shops and AR parts dealers keep this spring in stock specifically for Mk9 9mm AR bolt repairs, making it easy to match what your rifle was built with from the factory.
Ideal For:
The CMMG MK9 Firing Pin Spring 90BA419 is perfect for:
CMMG Mk9 9mm AR rifle owners who want to maintain factory-level reliability
Builders and gunsmiths assembling or servicing Mk9-based AR-9 platforms
Shooters experiencing light primer strikes or inconsistent ignition with older Mk9 springs
Anyone putting together a 9mm AR spare-parts kit for field repairs or long-term maintenance
If your 9mm AR is built on the Mk9 system, this is the correct firing pin spring to trust for long-term performance.
Key Specs – CMMG MK9 Firing Pin Spring 90BA419
Manufacturer: CMMG, Inc.
Model / Part Name: MK9 Firing Pin Spring
Manufacturer Part Number (MPN): 90BA419
UPC: 810046231340
Platform Compatibility: CMMG Mk9 / AR-9 9mm firing pin assemblies; listed for Kahr MK9 in some product descriptions (verify compatibility for your firearm before installation).
Material: Steel construction for durability and consistent spring tension
Type: OEM replacement firing pin spring (bolt/firing pin component)
Use Case: Repair, refurbishment, preventative maintenance, and reliability upgrades for MK9-based 9mm AR platforms
If you want to keep your CMMG MK9 or 9mm AR rifle running smoothly, the CMMG MK9 Firing Pin Spring 90BA419 (UPC 810046231340) is the exact, factory-spec part you need—reliable, affordable, and easy to install as part of your regular maintenance routine.
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