CMMG M16 Firing Pin Retainer — Mil-Spec Firing Pin Cotter Pin for AR-15/M16/M4 BCGs (Part 55BA4C5 | UPC 815835017341)
Don’t let a 50-cent part sideline your rifle. The CMMG M16 Firing Pin Retainer (often called a firing pin cotter pin) is the small, mission-critical component that secures the firing pin inside your AR-pattern bolt carrier group (BCG). Engineered to mil-spec dimensions and materials, Part #55BA4C5 delivers the precise spring tension and resilience needed for reliable, repeatable ignition—shot after shot, class after class. Whether you’re refreshing a high-round-count BCG, standardizing armorer spares, or building a new upper from the ground up, this is the tiny piece of insurance that keeps your rifle running.
Why Choose the CMMG 55BA4C5 Retainer
Mil-Spec Fit & Function: Correct wire diameter, leg length, and geometry for positive retention of the firing pin in standard AR-15/M16/M4 carriers.
Durable, Springy Steel: Heat-treated steel resists bending, cracking, and fatigue, even after repeated removal for cleaning.
Smooth Removal/Insertion: Consistent surface finish helps the retainer slide through the carrier bore without snagging or deforming.
Zero-Downtime Spare: Lightweight and inexpensive—keep two or three in your grip core or range kit so a lost pin never ends a training day.
CMMG Quality: Backed by one of the most trusted names in AR components and small parts.
What It Solves
Lost or distorted cotter pins that can cause misalignment or allow the firing pin to walk.
Hard starts during reassembly due to out-of-spec retainers that mushroom or bind.
Preventable malfunctions caused by worn retainers that no longer hold tension.
Compatibility
Platforms: Standard AR-15/M16/M4 direct-impingement carriers and most AR-pattern BCGs that use the M16-style firing pin retainer.
Calibers: Works across common small-frame AR calibers (5.56/.223, .300 BLK, 6.5 Grendel, etc.) that retain the M16-pattern BCG architecture.
Not for: Large-frame AR-10/SR-25 carriers that use different dimensions; verify your platform before purchase.
Quick Install (Field-Strip Friendly)
Unload and verify clear. Separate upper/lower; withdraw the BCG.
Remove old retainer. Pull the cotter pin from the carrier; slide out the firing pin.
Inspect & clean. Check the firing pin and channel; remove carbon and debris.
Reassemble. Insert firing pin fully, then push the CMMG 55BA4C5 retainer through the carrier until seated.
Function check. Confirm free firing pin movement and proper cam pin rotation before returning the BCG to service.
Pro Tip: Retainers are wear items. Replace whenever the legs deform, tension feels weak, or insertion is gritty. Pair replacements with fresh gas rings and a firing pin on your preventive-maintenance cycle.
Maintenance & Best Practices
Use proper tools: Needle-nose pliers or fingers—avoid prying that could deform the legs.
Lube lightly: A whisper of oil prevents corrosion but avoid pooling in the carrier bore.
Rotate stock: Mark and rotate retainers used for high-volume suppressed or short-gas rifles.
Specifications
Brand: CMMG
Part: M16 Firing Pin Retainer (cotter pin)
Part Number: 55BA4C5
UPC: 815835017341
Material: Heat-treated spring steel
Finish: Protective coating for corrosion resistance
Use: Secures firing pin within AR-pattern bolt carrier groups
What’s in the Package
Who It’s For
Builders & armorers maintaining fleets of ARs
Competitors & instructors who can’t afford preventable downtime
Prepared shooters who carry critical spares in every range bag
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Small part, major reliability. Keep your BCG locked together and your rifle in the fight with the CMMG M16 Firing Pin Retainer (55BA4C5)—the mil-spec cotter pin you should never be without.
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