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.

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Friday Links

Since we’re on the topic of triggers, here are a few links related to trigger design in addition to some general interest links.

The first is a PDF of the Remington-Walker trigger.

and an explanation of the Savage Accu-Trigger.

An interesting calculator to help figure gas pressures that may help in designing gas operated firearms.

Of course you’ve probably heard of making an AR-15 on a 3D printer by now.

Finally, while the explanations are basic, there are some excellent illustrations of firearms actions in the HowStuffWorks Machine Gun article.

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Wednesday Book Review: The Ultimate Thompson Book

The Ultimate Thompson Book
by Tracie L. Hill <copyright> 2009
Collector Grade Publications
ISBN: 0-88935-496-0

Ultimate Thompson Book

The Ultimate Thompson Book

The Ultimate Thompson Book is a heavy, handsome, and expensive tome that chronicles the entire development of arms under the direction of General John Thompson as well as the legacy of the company he helped create, Auto Ordnance. The book is aimed at historians and collectors, but is happily technical enough to warrant reading by those interested in firearms design as well.

In 1916 General John Thompson founded Auto-Ordance Corporation with the intention of creating an “Auto Rifle” – a self-loading rifle of such size and weight that it could serve in lieu of the standard rifle of the time, the M1903 Springfield.  His ace in the hole revolved around his acquisition of the rights to the “Blish principle” – what engineers today would call static friction, or ‘stiction’.  Thompson hoped that the Blish principle would allow the creation of a small, lightweight breech-locking mechanism that would make such a rifle possible.  To explore such possibilities he hired Theodore Eickhoff, Oscar Payne, and others to design such a firearm.  Several prototypes of the “Thompson Auto Rifle” were developed, but none of them faired satsifactorily in ordnance trials.  However, serendipitously, a byproduct of the rifle development was the discovery that the principle appeared to work exceedingly well in .45 ACP and the Thompson Submachine Gun, or “Tommy” gun was born.  Too late to see use in WWI, the Tommy gun was refined repeatedly as Thompson tried to sell the design to foreign militaries, law enforcement, and others.  Unhappily, the Thompson found its first enthusiasts within organized crime, an association that persists to this day.  At the beginning of WWII, Auto-Ordnance had sold a relatively small number of Tommy guns (fewer than the original manufacturing run of 15,000), but the war brought acceptance and demand.  During the course of the war, the original design was simplified for mass production and economy by engineers at Savage Arms, resulting in the M1 series Thompsons.  Post WWII saw demand for the Thompson diminish significantly, and as time passed, the design has faded in and out of popularity.  Today, nearly 100 years later, Thompsons are still in use.  Originals are much prized by collectors and shooting enthusiasts and several semi-automatic versions are still being manufactured.

The Ultimate Thompson book covers this history in exquisite detail.  The bulk of the book is organized chronologically, so intermingles biographies of key players, financing, politics, and of course, design.  The book is striking for its copious use of illustrations.  Having read many books and articles from this time frame, I thought the quality of the photographs and drawings exceptional.  The story is divided into many bite-size pieces, making the book easy to read in snippets as well as excellent as a reference.  The writing is excellent, and engaging, making the book a very enjoyable read.  Though it lacks an index, the Table of Contents is quite detailed and readily usable by anyone familiar with the story.

Of particular interest to the firearms designer, the book details the manner in which the original design goal (the Auto Rifle, remember?) was approached.  Early test setups are shown and described, as well as experimental mechanisms used during development.  Changes and improvements, as well as their motivations are also discussed.

While primarily of interest to collectors, the latter portion of the book details such things as appearances of the Tommy gun in movies, as toys, and in pop culture, as well as providing a list of known Thompson’s.

“The Ultimate Thompson Book” is one that truly lives up to its title, leaving no stone unturned, and detailing the Thompson firearms as never before.  It is an extraordinarily complete volume of the highest quality.  Valuation is always a subjective proposition, and someone with little interest in the Tommy gun may not enjoy this book as much as I did, but as a Tommy enthusiast, the hefty $150 I paid for my copy was money well spent.

5 Stars.

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Designing Triggers part 1: Simple Trigger Theory

The trigger is an integral part to any firearms design (various improvised ‘zip guns’ aside).  In its simplest form the trigger holds the firing pin, hammer or striker, hereafter referred to as the cocking piece, to the rear and releases it upon actuation.  It then resets to capture the cocking piece.  The trigger is sometimes also called into duty to perform auxiliary tasks such as cocking a hammer or striker, indexing a cylinder, and deactivating safety appliances such as firing pin blocks.

It is probably impossible to completely enumerate characteristics desirable in a trigger, and those characteristics may vary depending on the purpose of the firearm, but speaking in generalities, the following characteristics are desirable.

  1. Safety. The trigger mechanism should not be susceptible to accidental release by jarring, component fatigue, or other design flaw.
  2. Length of pull.  The trigger should have a comfortable length of pull that a normal human finger can traverse without strain.
  3. Weight of pull.  A trigger which requires a low force to activate is easier to use and is easier to activate without disturbing the alignment of the sights, resulting in greater functional accuracy.
  4. Crispness.  The ideal trigger, as the old saw goes, “breaks like a glass rod”.  This phrase is meant to convey that the ideal trigger accepts force, without apparent motion, up until the time when it gives way (releases) completely and without tactile feedback.  This aides the shooter by not telegraphing the moment the shot will break.
  5. Retraction is the ability of a trigger to fully reset after force has been applied, but the shot has not yet fired.  It would be dangerous to have a trigger that could remain ‘partially pulled’, resulting in a much lighter pull than expected.
  6. Consistency.  It is imperitive to accurate shooting that a trigger have the same weight and length of pull each time. Some firearms may have multiple ‘modes’, such as the first shot on a trigger-cocking firearm, or the single versus double action triggers on a revolver, but in a given ‘mode’ the trigger characteristics should be consistent.
  7. Ease of installation and removal, as well as maintainability
  8. Resistance to malfunctioning from introduction of debris or temperature variations.
  9. Price and manufacturability.  Modern manufacturing methods make light work of what was once difficult, but some features will increase the expense of a trigger beyond its value in some cases.
  10. Low parts count.  More parts means added complexity, as well as additional logistical support for manufacturers.  An additional piece may cost only a few cents more, but the tooling required to make it at such a price can be many thousands of dollars of overhead.

There are also factors which desirability may vary with purpose or user.

  1. Pre-travel.  Pre-travel is the amount of ‘slack’ that must be taken up before the full weight of the trigger begins.  Some pre-travel may give the user the ability to feel the trigger prior to discharge, it also increases the length of pull, which may add some safety margin.
  2. Overtravel.  Overtravel is the amount that the trigger is free to move after the point at which it activates.  In most applications, minimal overtravel is consdiered advantageous as it prevents any jarring caused by the trigger hitting a sudden stop after release.  With self-loading firearms, overtravel considered detrimental because it increases the reset distance.
  3. Reset.  Reset is the distance the trigger must travel forward (as pressure is released) before the trigger is ready to be fired again.  Reset is not a concern in single-shot firearms, but in self-loaders where a fast follow-up shot may be desirable, a short reset is preferred.
  4. Adjustability is frequently desirable in target triggers, but rarely so in military or hunting triggers, as adjustments introduce another potential failure point.

Finally there are features that are considered part of the trigger group, or trigger assembly, that may or may not be required depending on the firearm for which the trigger is intended:

  1. Disconnectors and secondary sears.  The term ‘disconnector’ in common use can mean any one of several things.  As used here, it is a device which disconnects the trigger from firing until the breech is closed.  In self-loading firearms this is necessary to prevent discharging a round while out of battery or allowing the hammer or striker to ‘follow’, resulting in an ineffectual strike (not firing when desired).  In a semi-automatic, a secondary sear remains engaged until the trigger has been released to the point of reset, before returning control to the trigger. In a fully-automatic, closed-bolt, firearm it then activates the hammer, or striker as soon as the breach is closed.  In other words, after firing is initiated by the trigger, it takes over the job of the trigger, timing each release to occur only after the breech is closed.
  2. Deactivating safety appliances.  The trigger may perform a secondary function of moving a firing pin block (for example) or other safety appliance.
  3. Indexing cylinders.  In a revolver, the trigger mechanism may be responsible for rotating the cylinder to the next chamber as a precursor to firing.
  4. Cocking the piece.  The trigger may be used to compress the spring that drives the hammer, striker,  or firing pin.

In this installment we will examine simple triggers: those which act directly to fire. We shall ignore for the moment the auxiliary functions as well as methods used to refine the quality of the trigger pull, and focus on the bare essentials. The triggers discussed in this part are suitable only for single shot or open-bolt firearms, but the principles involved apply to all triggers.

The basic trigger

A basic trigger is shown below.  It’s function should be immediately obvious upon inspection.  The striker at ‘A’ is held to the rear by the trigger ‘B’.  The trigger is held into it’s set position by trigger return spring ‘C’, which also returns the trigger to position after release.  The bevel at ‘D’ allows the sear on the striker to pass back over the trigger hook in order to re-engage the sear upon cocking.

basic trigger

Parts of a basic trigger.

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Friday links

FirearmsDesigner.com isn’t the only place on the web to find good information on gunsmithing and firearms design, so we’ll try to post up other useful sites on Fridays.

Dan Lilja is a barrel maker of excellent repute and he’s posted a small library of excellent articles on his site.

Although we’ve covered the topic here, here’s another take on making chamber reamers.

The folks at Forgotten Weapons dig up some pretty interesting bits about historical firearms development.

Some insight from a design engineer at Kel-Tec.

If you need raw forgings to make an AR-15 it appears that Tactical Innovations still has stock.

Orion’s Hammer posted an excellent and informative article on the design of blow-back actions.

We’ll try to keep your reading list fueled with more links next week!

 

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