A detent is a generic term used to mean a device, mechanism (in this case, a pin) which essentially keeps another part from moving, forces it to move in one direction, or holds one part in place.

The AR-15 has several detent pins (and detent springs) to allow the firearm to function properly. For example, in the lower parts kit, you’ll find:

A detent is a generic term used to mean a device, mechanism (in this case, a spring) which essentially keeps another part from moving, forces it to move in one direction, or holds one part in place.

The AR-15 has a couple of detent springs to allow the firearm to function properly. For example, in the lower parts kit, you’ll find:

  • Pivot Detent Spring
  • Takedown Detent Spring

The lower parts kit for an AR-15 usually contains all of the pins/detents and springs necessary to complete the lower receiver.

Some lower parts kits will omit the fire control, safety, and pistol grip – or may exchange/upgrade these items for better-than-mil-spec quality parts.

Traditionally, a complete lower parts kit contains:

    • Bolt Catch,
    • Bolt Catch Plunger,
    • Bolt Catch Roll Pin,
    • Bolt Catch Spring,
    • Buffer Retainer,
    • Lower-Parts-Kit

    • Buffer Retainer Spring,
    • Disconnect,
    • Disconnect Spring,
    • Hammer,
    • Hammer Pin,
    • Hammer Spring,
    • Magazine Catch,
    • Magazine Catch Button,
    • Magazine Catch Spring,
    • Pistol Grip (A2 style),
    • Pistol Grip Lock Washer,
    • Pistol Grip Screw,
    • Pivot Pin,
    • Pivot Pin Detent,
    • Pivot Pin Detent Spring,
    • Selector,
    • Selector Detent,
    • Selector Detent Spring,
    • Takedown Pin,
    • Takedown Pin Detent,
    • Takedown Pin Detent Spring,
    • Trigger,
    • Trigger Guard,
    • Trigger Guard Roll Pin,
    • Trigger Pin,
    • Trigger Spring

The Takedown Pin is located at the rear of the lower receiver, near the buffer tube, and assists the pivot pin in securing the upper and lower receivers together.

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Exposing the interior of the AR-15 by removing the Takedown Pin

Once removed, the takedown pin allows the upper receiver to pivot on the front pin, while exposing the interior parts of the AR-15.

Takedown pin

The takedown pin can be installed in conjunction with the takedown detent pin and takedown detent spring