Lower Parts Kit Installation: Building a Stripped Lower Without Losing a Detent

Assembling an AR-15 lower is the easiest job in gunsmithing, which is why so many people do it badly. Nothing in it needs a lathe or a press, but four tiny springs and detents will launch across the room the moment you look away, and two pins will scratch the receiver if you drive them without care. Done in the right order with a bench block and a little tape, the whole lower goes together in under an hour. This guide sets out that order, the tools, and the checks that catch a problem before the range does.

Tools

A polymer bench block or a block of hardwood with holes drilled for the pins, a set of roll-pin punches including a 3/32 and a 1/8, a brass or nylon hammer, a small flat screwdriver, masking tape, a bit of grease, and for the buffer tube a castle-nut wrench and a torque wrench. A pivot-pin installation tool costs a few dollars and saves the most commonly lost detent. Needle-nose pliers help with springs. Lay every part out on a towel before you start and count the detents: there should be four.

Step one: the magazine catch

Drop the magazine catch into its slot on the left side of the receiver, put the spring over the threaded shaft on the right side and the button over the spring. Press the button in with a punch and turn the catch until its shaft sits flush with or just below the face of the button. Check that a magazine locks and drops cleanly.

Step two: the bolt catch

Tape the left side of the receiver around the bolt-catch boss to protect the finish, because this is the pin that scratches. Put the bolt-catch spring and plunger into their hole, hold the bolt catch in place, and drive the roll pin through with a 3/32 punch from the rear, keeping it square. A dedicated bolt-catch punch with an offset makes it easier. Test that the catch pivots and springs back.

Step three: the trigger guard

Many lowers have an integral guard and skip this. If yours has a separate guard, seat the detent end first, then support the receiver's ears on the bench block and drive the roll pin through the rear. Unsupported ears break here; support them.

Step four: the pivot pin

This is the detent everyone loses. Put the pivot-pin detent spring into its hole at the front of the receiver, then the detent. Use the installation tool, or a clevis pin with a slot, to hold the detent down while you slide the pivot pin in from the left with its groove toward the detent. Without the tool, cover the area with a clear bag so the detent lands in the bag when it launches.

Step five: trigger and hammer

Fit the trigger spring on the trigger with its legs pointing forward and resting on the receiver floor, drop in the trigger with the disconnector and its spring in the trigger's slot, and push the trigger pin through from the left. Then the hammer: its spring's legs point back and rest on top of the trigger pin, which is what stops the trigger pin walking out. Push the hammer pin through with the hammer cocked back. Both pins should slide with light pressure; a lower that needs the pins hammered is a lower with a burr, and a pin driven hard will crack the polymer of a cheap trigger. A drop-in trigger replaces this whole step with a cassette and two pins; the maker's instructions apply.

Step six: safety selector and grip

Put the safety selector through from the left with the hammer cocked. Put the selector detent in its hole under the grip from below, spring behind it, then fit the grip over the spring and tighten the grip screw, which captures the detent. Check that the selector clicks between safe and fire and that with the selector on safe the trigger does not release the hammer.

Step seven: the buffer tube, takedown pin and stock

Thread the buffer retainer and its spring into their hole at the rear, then start the buffer tube into the receiver threads. As it screws in, it captures the buffer retainer; stop when the tube's lip holds the retainer down but does not crush it. Fit the takedown-pin detent and spring into their hole at the rear of the receiver, slide the takedown pin in from the right, then push the end plate over the spring and tighten the castle nut. Torque the castle nut to around forty foot-pounds and stake it into the end plate with a centre punch so it cannot back off. Slide the stock on, drop in the buffer spring and buffer, and the lower is built.

The function check

Selector on safe, pull the trigger: the hammer must not fall.
Selector on fire, pull the trigger: the hammer falls. Hold the trigger back, cock the hammer, release the trigger: you should hear and feel the disconnector hand the hammer to the sear with a click, not a second fall.
Cock the hammer, push the bolt catch: it should hold. Magazine in and out cleanly.
Pivot and takedown pins snap into their detents at both positions.

Common mistakes

Trigger spring legs behind the trigger instead of in front give a trigger that will not reset. A hammer spring installed backwards gives light strikes. A castle nut left loose lets the buffer tube unscrew a quarter turn and the rifle stops cycling. A detent left out of the takedown pin lets the pin walk out under recoil. Each is a five-minute fix on the bench and a ruined afternoon at the range.

The lower is built; now match the buffer to the upper and read the controls guide before you swap the selector. Browse parts kits, triggers and lowers in the catalogue.

General information, not legal advice. Current as of September 2026. Laws change; check the official source before you act.