Short answer: unload the rifle and take the upper off, ease the hammer down, push out the hammer and trigger pins, then fit the new trigger with its springs the right way round and do a function check before you shoot. A standard trigger takes 20 to 30 minutes the first time; a drop-in trigger is quicker.
Safety first: remove the magazine, lock the bolt back and check that the chamber is empty, and keep ammunition out of the room while you work. If anything in the function check fails, don't fire the rifle: recheck the springs or take it to a gunsmith. Your trigger maker's instructions come first.
Tools
- A 5/32-inch (or 3/16-inch) pin punch, plus a second straight punch or slave pin
- A small nylon or brass hammer
- A lower receiver vise block (helpful, not required)
- A towel under the receiver so springs and pins don't get lost
Step by step
- Unload and separate the upper. Push out the takedown and pivot pins and lift the upper off.
- Ease the hammer down. Put the safety on FIRE, hold the hammer with your thumb, pull the trigger and lower the hammer gently. Never let it slam into the receiver.
- Remove the hammer. Push out the rear (hammer) pin and lift out the hammer and its spring.
- Remove the trigger. With the safety still on FIRE, push out the front (trigger) pin. The trigger, disconnector and their springs come out together.
- Assemble the new trigger. Fit the trigger spring so its legs point forward and press on the floor of the receiver. Put the disconnector spring in the trigger, wide end down, and set the disconnector on top. Hold everything in line with a slave pin or punch.
- Install the trigger. Drop the trigger group into the receiver and push the trigger pin in from one side, letting it push the slave pin out the other.
- Install the hammer. Fit the hammer spring with its looped part behind the hammer. The spring legs rest on top of the trigger pin, one leg in the pin's groove. Press the hammer down and push the hammer pin in.
- Function check, then put the upper back on.
Function check
| Step | Do this (upper off or chamber empty) | This must happen |
|---|---|---|
| 1 | Cock the hammer, safety on SAFE, pull the trigger | The hammer does not fall |
| 2 | Safety on FIRE, pull the trigger (catch the hammer) | The hammer falls |
| 3 | Hold the trigger back, cock the hammer, then slowly let the trigger go | A click, and the hammer stays cocked |
| 4 | Pull the trigger again | The hammer falls |
Drop-in triggers
A drop-in trigger replaces the hammer, trigger and springs with one housing, so steps 5 to 7 become "drop it in and push two pins through". Many come with anti-walk pins, and some have set screws that seat the housing against the receiver; follow the maker's instructions and do the same function check.
Mistakes that cause most problems
- Hammer spring legs under the trigger pin instead of on top of it: the hammer has no power.
- Trigger spring facing backwards: the trigger won't return or reset.
- Disconnector spring upside down: no reset or doubling.
- Letting the hammer slam into an empty receiver: it can crack the receiver.
- Wrong pin size: most AR-15 lowers use small .154-inch pins, some older ones large .170-inch pins.
Choosing a trigger? Compare AR-15 triggers, two-stage triggers and drop-ins, and read our trigger types compared and best AR-15 triggers. The AR-15 builder keeps your trigger with the rest of your parts.
Want to know why the disconnector and its spring matter? See how an AR-15 trigger works.
Frequently asked questions
Do I need a gunsmith to install an AR-15 trigger?
No. With a pin punch and some patience most people can do it at home. If the function check fails and you can't find why, a gunsmith can fix it quickly.
How long does it take to install an AR-15 trigger?
About 20 to 30 minutes for a standard trigger the first time, and 10 minutes for most drop-in triggers.
Small pin or large pin trigger?
Most AR-15 lowers use small (.154-inch) pins. Some older lowers use large (.170-inch) pins, and AR-10 and some 9mm lowers differ. Measure your pins before you buy.
Do I have to remove the pistol grip?
No. The grip only holds the safety selector's spring, and the trigger comes out without touching it.
Why won't my new trigger reset?
Usually the disconnector spring is upside down or the hammer spring legs are under the trigger pin. Recheck both, then do the function check again.
Sources: trigger makers' instructions and the NRA Shooting Illustrated guide to AR lower trigger installation. Your trigger's own instructions always win.
