AR-15 Trigger Install and Tuning: From the Parts Bag to the Function Check

The trigger is the AR-15's most-replaced part and the one most often installed wrong. It is a ten-minute job with two pins, and the ways it goes wrong are few and well known: a spring in backwards, a pin that walks, a cassette that rocks, a spring kit that buys a light pull with light strikes. This guide takes a mil-spec group and a drop-in cassette through installation, the function check that catches mistakes, and the tuning that is worth doing and the tuning that is not. It ends with the one Florida rule that touches triggers.

What is in the bag

A mil-spec trigger group is a hammer with its spring, a trigger with its spring, a disconnector with its small spring, and two pins. A drop-in cassette is all of that assembled in an aluminium housing with, usually, anti-walk pins and a pair of set screws. Both install with the same two pin holes in the lower. Check the pin size before you start: nearly all lowers take .154-inch pins; a few older Colt lowers take .170-inch large pins and need a large-pin trigger.

Removing the old trigger

Clear the rifle, separate the upper, and cock the hammer. Push the hammer pin out from left to right with a punch, catching the hammer and its spring. Push the trigger pin out the same way and lift out the trigger, disconnector and springs. The selector stays put.

Installing a mil-spec group

Fit the trigger spring on the trigger with its legs pointing forward and resting on the receiver floor; legs behind the trigger give a trigger that will not reset. Drop the disconnector spring into the trigger's well with its wide end down, set the disconnector on it, lower the assembly into the receiver and push the trigger pin through from the left, holding the disconnector down. Fit the hammer spring with its legs pointing back and resting on top of the trigger pin, which is what keeps the pin from walking, and push the hammer pin through with the hammer cocked. Pins should slide with light pressure; a pin that needs hammering is finding a burr.

Installing a cassette

Lower the cassette into the receiver with the selector in the position the maker specifies, seat it flat on the floor, and push the two pins through. Fit the anti-walk screws from the far side and snug them. Tighten the set screws in the housing evenly until the cassette cannot rock, then a fraction more, with a drop of thread locker. A cassette that rocks gives an inconsistent break and can fail to reset; a cassette bowed by over-tightened screws binds.

The function check

Selector on safe, press the trigger: the hammer must not fall.
Selector on fire, press: the hammer falls. Hold the trigger back, cock the hammer, release the trigger slowly: a click as the disconnector hands off, not a second fall.
Cock the hammer and move the selector: it must move freely.
Rap the butt on a padded floor with the hammer cocked and the selector on fire: the hammer must not fall.
Reassemble, cycle the action ten times pressing the trigger each time, and watch the pins.

Tuning that is worth doing

Choose the trigger for the weight you want rather than lightening one after the fact. An enhanced mil-spec group with polished sears and a coating gives four to four and a half pounds with the grit gone; a drop-in match trigger gives two and a half to three and a half; a two-stage gives a light break with a wall. Flat shoes lower the felt weight for short fingers. Anti-walk pins on a cassette and a properly seated hammer spring on a mil-spec group are tuning too, because a trigger whose geometry moves is a trigger whose weight moves.

Tuning that is not

Reduced-power spring kits drop a pound of pull for a few dollars and cause light strikes on hard primers, especially in the cold; a rifle that hunts hogs at dawn in January does not want one. Stoning or filing sear surfaces takes the engagement below what the maker designed and is how doubling rifles are made; a rifle that fires more than once per press is a broken and unlawful firearm until it is fixed. Buy the weight, do not cut it.

Selectors and cassettes

A cassette's housing sits where the safety selector passes through the receiver, and a few ambidextrous selectors with a wide centre section will not clear it. Most cassette makers list which selectors fit; a standard mil-spec selector fits every cassette. Install the cassette with the selector already in the receiver and set to the position the instructions specify, and if the selector binds after the pins are in, the profile is the cause and a different selector is the fix, not a file.

The range test

Fifty rounds of the ammunition you actually use, checking the pins after the first ten and at the end. Light primer strikes point to a weak hammer spring or a firing-pin channel packed with oil; failures to reset point to a rocking cassette, a walked pin or a trigger spring in backwards. All of it is a five-minute fix on the bench.

The Florida rule

Florida, like federal law, prohibits any device that converts a semi-automatic to fire more than one round per trigger pull. Every trigger we sell fires one round per press, and we sell nothing that changes that. Beyond that, Florida sets no rule on trigger weight, type or origin, and a trigger ships to your door with no check and no wait. The law guide covers the rest; the Glock guide and the 1911 guide cover the pistol bench. Browse triggers and uppers in the catalogue.

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