Making a Hammer-Fired Trigger Module for Prototyping

One of the things I’d like to attempt as this site builds up to actual design of firearms, is to create some modular pieces that can be used when other parts of the design are the point of interest. As an example, relevant to today’s article, if you wish to experiment with a new breech-locking mechanism, the design and assembly of a trigger is a nuisance – you need a trigger, but the time and energy spent creating it are a distraction from the thing you are really trying to test.

To that end, a trigger module, or cassette, is a desirable unit to have kicking around in one’s toolbox. In reality, several trigger cassettes might be useful: one to activate a hammer and one to release a striker. Today, we’ll look at a simple hammer-type trigger cassette utilizing AR-15 trigger components.


By utilizine off-the-shelf components, we’re relieved from having to work out a trigger mechanism when the trigger design is not the subject of interest. We also have some assurance that the design is robust and tested, so that it not confound or complicate our attempts to test other aspects of the firearm design.

Design Criteria

We want a trigger mechanism that utilizes AR-15 trigger components (off the shelf) for use in to-be-designed firearms. Because the system is hammer-fired, the hammer must protrude above the system far enough to impact a firing pin in another module. It must also protrude enough that it can be cocked by motion of the bolt in the action module.

The rest of the design is fairly straightforward: we copy the hole locations and body dimensions of the relevant parts of an AR-15. It turns out that something very close to the desired dimensions is available in stock material: 1″ x 1.5″ rectangular steel tubing with 1/8″ walls.


Raw stock, ready to go.

Raw stock, ready to go.

A piece of material ~3.5″ long is used. A 3/8″ hole for the safety is drilled as shown in the drawing (drawing at end of article).


Followed by holes for the hammer and trigger pins.


The part is turned ‘up’ and holes are drilled all the way through (while we have a drill chuck in the mill) at the endpoints of the trigger slot. The top is then milled open.


With the top machined off, the trigger slot is machined, and the cassette body is done.


The completed unit accepts an AR trigger group as-is, but further refinements might include longer pins (the tube is slightly wider than an AR-15), addition of a detent for the safety, and addition of a pistol grip and trigger guard.


When contemplating use in a new design, mounting holes may be created allowing the cassette to be pinned or screwed onto an action, as required.

PDF of trigger cassette drawing:  AR-Trigger-Cassette


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