AR-15 Mil-Spec Hammer with J-Spring — Hardened Steel, Direct Replacement, Semi-Auto (UPC 859819007577)
Restore crisp ignition and reliable cycling with the AR-15 Mil-Spec Hammer with J-Spring—a precision, drop-in replacement sized to U.S. mil-spec dimensions for most AR-15 pattern semi-automatic lowers. Built from hardened tool steel and finished for wear resistance, this hammer delivers consistent primer strikes and smooth interaction with standard triggers, disconnectors, and sears. Whether you’re refreshing a high-round-count carbine, completing a lower-parts build, or keeping spares on hand, this hammer is the no-nonsense choice for duty, training, and range rifles.
Why This Mil-Spec Hammer
Mil-Spec geometry: Interfaces correctly with standard AR-15 fire-control groups, ensuring proper sear engagement and reliable reset.
Integrated J-spring: Factory-installed J-spring retains the hammer pin and improves pin stability without extra hardware.
Durable construction: Hardened steel with protective finish resists peening and galling under high round counts.
Direct replacement: Drop-in fit with most 5.56/.223 and caliber-variant AR-15 lowers using mil-spec FCG components.
Compatible with common LPKs: Plays nicely with standard trigger, disconnector, hammer/trigger pins, and springs from leading lower-parts kits.
Great for maintenance kits: Affordable, proven part to keep in your armorer’s spares for field fixes and overhauls.
Pro Tip: Pair this hammer with new hammer and trigger springs during a refresh. Replacing springs together helps maintain consistent pull weight and primer ignition.
Key Features at a Glance
Platform: AR-15 (semi-automatic; mil-spec FCG)
Part: Hammer with J-spring installed
Material: Hardened steel
Finish: Wear-resistant protective coating (color may vary by lot)
Pins: Uses standard 0.154" hammer/trigger pins (not included)
Compatibility: Works with most mil-spec lowers and standard single-stage triggers (not for Colt-style .170" large-pin receivers)
UPC: 859819007577
SKU: 859819007577
Performance You’ll Notice
Reliable primer strikes: Correct geometry and consistent energy delivery reduce light-strike issues with typical 5.56/.223 loads.
Smooth cycling & reset: Proper engagement surfaces support a clean break (trigger-dependent) and positive reset.
Longevity under use: Hardened contact faces stand up to high-volume training and carbine classes.
Ideal Use Cases
Lower-parts kit build-outs and first-time rifle assemblies
Preventive maintenance for training, range, and home-defense carbines
Spare/field kits for armorers, instructors, and competitors
Installation Overview (Gunsmithing Basics)
Unload & verify clear. Separate upper/lower; work on the lower receiver only.
Remove old components. Drift out 0.154" pins, capturing springs as you go.
Install new hammer. Position the J-spring hammer, route the hammer spring legs properly over the trigger pin, and insert the hammer pin.
Function check. With an unloaded lower, verify safety engagement, reset, and disconnector operation before reassembly.
Final inspection. Confirm the hammer doesn’t follow when the trigger is released and the safety works in all positions.
If you’re unsure about AR fire-control installation, consult a qualified gunsmith.
Compatibility Notes
Pins: Designed for small-pin 0.154" AR-15 lowers; not compatible with older 0.170" large-pin receivers.
Triggers: Intended for mil-spec single-stage triggers. May not be compatible with some match/aftermarket or binary systems—check the trigger maker’s guidance.
Calibers: Works across common AR-15 calibers (5.56/.223, 300 BLK, 6mm ARC, 6.5 Grendel, etc.) using standard FCGs.
Care & Maintenance
Lubricate lightly: Add a drop of oil to pivot points and sear/hammer engagement per AR best practices.
Inspect regularly: Watch for peening or abnormal wear; replace springs at interval for consistent performance.
Keep pins seated: The integrated J-spring aids retention, but verify pin walk after high-round sessions.
Specifications
Type: AR-15 Mil-Spec Hammer w/ J-Spring
Material: Hardened steel
Pin Size: 0.154" (standard small pin)
Finish: Protective coating
UPC: 859819007577
SKU: 859819007577
Safety & Legal
Always follow firearm safety rules and verify unloaded before any work.
Ensure compliance with federal, state, and local laws regarding firearm assembly and modifications.
After installation, perform a complete function check and range test with appropriate ammunition.
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FAQ
Q: Will this fit my Colt AR with large pins?
A: No. This hammer is for 0.154" small-pin lowers. Large-pin (0.170") receivers require a different part.
Q: Does it include pins and springs?
A: The hammer includes the J-spring only. Hammer/trigger pins and coil springs are typically part of a lower-parts kit and sold separately.
Q: Compatible with match triggers?
A: It’s intended for mil-spec single-stage triggers. Consult your match-trigger manufacturer for compatibility.
Q: Will it reduce my trigger pull weight?
A: Pull weight is primarily determined by the trigger/sear geometry and springs. This hammer maintains mil-spec characteristics.
Q: Do I need a gunsmith to install it?
A: Many owners install mil-spec hammers at home, but if you’re unfamiliar with AR FCGs, a qualified gunsmith is recommended.
Get back to reliable, repeatable ignition with the AR-15 Mil-Spec Hammer with J-Spring (UPC 859819007577)—the durable, drop-in solution for fresh builds, overhauls, and armorer kits.
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