Drop In Trigger vs. Two Piece Trigger and other Considerations

Short Story: The trigger is arguably one of the most important components on an AR, and there are a lot of options to choose from.

Let’s first discuss the purpose of the trigger assembly.

Trigger Purpose

The trigger and associated components are designed for one purpose: put the operating system into action. Commonly called the “fire control group”, the trigger components set off the chain reaction of events that puts in lead down range.

The application of a trigger may demand certain qualities (e.g. light pull weight, short reset, etc.), but the purpose remains the same.

Next, let’s explore the construction of a trigger assembly.

Trigger Construction

The trigger is generally comprised of 6 functioning parts:

1. Trigger: The trigger is a shooter’s connection to the fire control group. It is via this part that we fire a gun. On the front face of the trigger, we find the trigger sear, which engages a notch on the hammer to hold it in the cocked position.
2. Hammer: The hammer is the part that actually sets off the ignition sequence. When the trigger is pressed to the rear, the trigger sear slips out of the sear notch on the hammer, releasing the hammer and allowing the hammer spring tension to drive the hammer forward.
3. Disconnector: In semi-automatic firearms, this part hooks onto the hammer as the hammer is pushed back into cocked position by the reciprocating bolt carrier. The disconnector holds the hammer back, until the trigger is released and resets, at which point the trigger sear falls into the sear notch on the hammer.
4. Trigger Spring: This is responsible for the “pull weight” of your trigger. A spring with lower tension results in a lighter trigger pull. A spring with a higher tension results in a heavier trigger pull.
5. Hammer Spring: This is responsible for driving the hammer forward during the cycle of fire. The heavier this spring, the harder the hammer will strike the firing pin.
6. Disconnector Spring: This spring gives a bit of give to the disconnector as the hammer is pushed rearward by the reciprocating bolt carrier group. Without this spring (i.e. if the disconnector was directly linked to the trigger), the trigger would be bumped forward by the hammer hook when the hammer is pushed rearward.

Two additional components are important to consider as well:

• Safety Selector Switch: When in the “safe” position, the barrel of the safety selector prevents the trigger from traveling far enough for the sear to disengage the hammer. When the safety selector switch is rotated to the “fire” position, a notch in the safety selector barrel allows the trigger to travel far enough to release the hammer.
• Trigger/Hammer Pins: These pins hold the trigger and hammer in the lower receiver and in the proper positions relative to one another.

Next, let’s examine the operation of a trigger.

Trigger Operation

The following illustrates the sequence of events when a cocked trigger group is activated.

1. The shooter switches the safety selector to the “fire” position. The notch in the safety selector rotates to allow the trigger to travel.
2. The trigger is pressed rearward by the shooter, against the tension of the trigger spring. The trigger pivots rearward around the trigger pin. The front of the trigger (sear) rotates downward.
3. The sear surface of the trigger clears the hammer’s sear engagement notch.
4. The hammer is driven forward by the tension loaded in the hammer spring. The hammer pivots forward around the hammer pin.
5. The hammer strikes the back of the firing pin, transferring the kinetic energy into the firing pin. The firing pin moves forward in the bolt and the tip strikes the primer of the waiting cartridge, igniting the primer and powder. The firing sequence continues until the pressurized gas travels through the gas system and drives the bolt carrier rearward.
6. As the reciprocating bolt carrier group moves rearward, it pushes the hammer. The hammer rotates around the hammer pin and the force re-loads the hammer spring tension.
7. The hammer hook strikes the disconnector hook. The disconnector rotates around the trigger pin and compresses the disconnector spring, allowing the hammer hook to travel past the disconnector hook.
8. The compressed disconnector spring pushes the disconnector upward. As the hook on the hammer passes the hook of the disconnector the hooks engage one another. The hammer is held in place until the trigger is released and resets.
9. The shooter releases pressure from the trigger, allowing it to travel forward under the tension of the trigger spring. The trigger rotates around the trigger pin.
10. The movement of the trigger forces the disconnector hook to release the hammer hook. The hammer rotates forward around the hammer pin.
11. As the hammer rotates forward, the sear notch on the hammer engages the sear on the trigger. The hammer is held in place until the trigger is pressed again.
12. The shooter switches the safety selector to the “safe” position. The barrel of the safety selector rotates, and the notch is repositioned such that the trigger can no longer travel.

This video by AWI perfectly illustrates what is happening in this sequence:

Next, let’s explore the parameters that apply to the trigger group.

Trigger Design Parameters

Number of Stages

Short Story: Single stage is better for lighter weight precision triggers; two stage is better for higher weight combat triggers.

Single Stage

A single stage trigger is just as it sounds. When you apply pressure to the trigger, there should be little creep or uptake (Mil-Spec triggers can be a bit mushy and sloppy). You immediately reach a wall of resistance and, when the appropriate amount of pressure is applied to the trigger, the sear disengages the hammer and the firing cycle begins.

If you need a trigger with no take-up or travel, a short and fast pull, a light and crisp break, and a fast reset, you want a quality single stage trigger. These characteristics are perfect for a dedicated precision rifle, where you will not be rushed, under excessive stress, or moving around.

If you will be moving around or under stress, you don’t want a super light, single stage trigger. Stress, motion, and chaos do bad things to fine motor control. A light trigger increases the likelihood of accidental (or negligent) discharge. If you have your heart set on a single stage trigger for these scenarios, go with a trigger pull of at least 4.5 lbs.

Two Stage

Unlike a single stage trigger, a two stage trigger is not all or nothing. The first stage of a two stage trigger is light weight pre-travel, or “slack”, that must be taken up to reach the wall of resistance before a trigger breaks. The second stage is from the onset of that wall until the break.

With a two stage trigger, the shooter can always change his or her mind and back off of the trigger before it breaks. This is useful in stressful situations.

When the shooter follows through and the trigger breaks, the shooter has an option: he or she can release the trigger completely, allowing it to fully reset, or release it to the edge of the first stage. This is particularly useful for quick follow-up shots, which is particularly useful in defensive, combat, and competitive scenarios.

If you want a gun that will function well under chaotic and stressful situations, a two stage trigger is probably best. A quality two stage trigger is also resilient enough to offer great performance for precision shooting, as well. It is a better “all around” option.

Trigger Assembly

Short Story: Two piece triggers are best for heavier two stage pulls; drop-in triggers are best for lighter single stage pulls.

Two Piece

A two piece, or “Mil-Spec”, trigger actually comes in 6 pieces; we suppose “six piece trigger” didn’t sound as elegant. With a two piece trigger, the trigger, hammer, disconnector, trigger spring, hammer spring, and disconnector spring are all separate. They must be assembled (properly) and installed in the lower receiver.

Two piece triggers come in both single stage and two stage variants. Two piece triggers are king for two stage needs. Where light, crisp, and consistent single stage trigger pulls are desired, we recommend that you consider a single stage drop-in trigger over a single stage two piece trigger, as they are more precise and controlled instruments.

The pull weight of a two piece trigger is only adjustable by swapping out the trigger spring for a different weight spring.

If you are considering a two piece trigger, please check out our article on hammer/trigger pins.

Drop-In

Drop-in, or cassette, triggers have all of the components pre-installed in a metal housing. True to the name, they literally drop into the lower receiver and are secured in place by the trigger and hammer pins.

Drop-in triggers come in both single stage and two stage variants. That said, drop-in trigger really shine as single stage triggers, where consistency demands tight tolerances. Because the trigger components are pre-assembled into a fixed metal housing, they are very robust from an installation error perspective. The position of trigger and hammer pins will have no effect on the alignment of the sear, so installation will have no effect on the performance of the trigger itself. Additionally, some drop-in triggers feature sealed bearings for the trigger and hammer, so the operation is even more consistent under adverse conditions. In our opinion, there really isn’t a reason to select a two stage drop-in trigger; the two piece alternative is spectacular for scenarios where a two stage pull is appropriate.

Drop-in triggers may include functionality to adjust the trigger pull weight via an internal adjustment screw. Whereas the pull weight of two piece triggers can be adjusted in big steps via different nominal weight trigger springs, drop-in triggers with adjustable pull weight are finely-adjustable in the nominal pull weight range. This adjustability means that you don’t have to settle for either 3.75 lbs. or 4.5 lbs.; if you want a 3.925 lb. trigger, you can have it with the twist of a screw.

It is worth noting that drop-in triggers are often intricate and tend to have more points of failure. Depending on the design, it may not be as robust to rough handling or heavy soiling.

If you are considering a drop-in trigger, please check out our article on hammer/trigger pins.

Pull Weight

Short Story: If you want a lighter pull and want to be able to tune the pull weight to your preference, adjustable is the way to go; adjustability at higher pull weights is not really useful for most.

Fixed

This doesn’t require much explanation. Fixed pull weight is static. The only way to change the pull weight is to change the trigger spring for a lighter or heavier spring.

Fixed is particularly appropriate where your pull weight is combat-appropriate (i.e. heavier). The difference between 4.5 lbs. and 4.7 lbs. is imperceptible and, therefore, not particularly important.

Adjustable

Triggers with an adjustable pull weight typically have a screw that can be manipulated to change the trigger spring tension. Adjustable triggers will have a fixed range of weights, with the ability to adjust within that range, either incrementally (i.e. stepped or ratcheting screw) or continuously.

Adjustable triggers are most appropriate for lighter pull weight triggers, where a change in weight has a greater impact on the feel of the trigger. That said, if you find a fixed weight trigger that has a pull weight that meets your needs, you don’t need an adjustable trigger; it becomes a matter of preference and taste.

Trigger Shape

Short Story: The shape of the trigger is entirely a shooter preference; there is no inherent benefit to one over the other.

Curved

Curved triggers have been the standard for hundreds of years, which is a pretty good reason to consider one.

Curved triggers are generally perceived to be more comfortable than flat triggers. The shape is a more natural mate for a finger. Because it curves back, it also sits farther back in the trigger guard, which means less of a reach for the shooter and more room for a gloved finger.

Flat

Flat, or straight, triggers are a relatively new option in the AR trigger world, and have found a home among competitive shooters.

Flat triggers are said to feel more consistent than curved triggers. There is less perceived difference in feel of a flat trigger at any given point on the trigger shoe, when compared to a curved trigger, which can feel very different, depending on where you place your finger on the curve.

PB Picks

PB’s Favorite Brands

We recommend specific brands of triggers. The brands we recommend are not novel. Most bloggers, competitors, and experts recommend some mix of the same brands. The reason is simple: BECAUSE THEY ARE THE BEST. We are not the first ones to realize this. We won’t be the last.

PB’s Trigger Picks

Among our preferred trigger brands, there are a few specific models that work very well in the appropriate applications. Below are the criteria that we use for judging best application:

PB Picks

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