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How “Automatic” Pistols Really Work by Adam Firestone

It’s no secret books, TV and social media inaccuracies about weapons annoy Adam Firestone [1]the way misplaced apostrophes annoy grammar nerds. Read on to find out why “gas-operated blowback pistol” is not a phrase you want to bring up when Adam’s around.


If you’ve been reading this column for any length of time, you know I have hot buttons – and most of them center on technical inaccuracies committed by well-meaning writers.  (I know – serious first world problems…)  One of the most sensitive of my tripwires has to do with faulty descriptions of pistol operating mechanisms.  Seriously.  In fact, if you’re in dire need of cocktail party entertainment, invite me, and then have someone wander over and say “gas-operated blowback pistol.”  I promise that the results will be entertaining for your guests and embarrassing for me.  And so, in an effort to cut down on the amount of public humiliation I suffer while at the same time increasing the accuracy of your technical writing, I’m offering a bit of a primer on how semi-auto pistols really work…

It is commonly expected that the operating mechanism of a semi-automatic, or self-loading pistol will do a number of things.  It is intuitive that the pistol must have reciprocating parts to manage the insertion of the fresh cartridge into the chamber, the firing of the cartridge, the extraction of the spent casing from the chamber, the ejection of the spent casing, and the insertion of a new fresh cartridge into the chamber.  What is less intuitive is the extent of the firearm’s mechanism that goes into solving the problem of locking and unlocking the breech block.

Pistol cartridges will generate pressures measured in the tens of thousands of pounds per square inch when fired.  These pressures want to force the head of the case, and consequently, the breech block mechanism, be it slide, toggle, or bolt, to the rear.  As a result, there must be some sorts of mechanism in place to hold the cartridge inside the chamber of the firearm during firing.  The engineering problem is compounded by the fact that the high pressure event inside the chamber/barrel only lasts a very short amount of time, not more than .001 seconds, and thus, the mechanism must be designed to use the available energy within a very short, precise interval.


If the breech or slide of the pistol is heavy enough, it would not move rearward far enough to allow any damage to occur during the short high pressure interval.  In fact, if the mass of the breech mechanism is chosen carefully, no mechanical locking mechanism would be required; the pressure on the cartridge case would be just enough to give the breech mechanism enough movement so that the breech would open after the cartridge is fired, allowing for safe extraction and ejection of the fired case.  If a spring is placed behind the breech mechanism to ensure that it closed once more, the forward movement could be harnessed to drive a fresh cartridge into the chamber.

This is the breech mechanism that is used in many small self-loading pistols that fire medium to low powered cartridges.  It is generally acknowledged that the most powerful cartridge a small, blowback operated pistol can use is the 9x18mm, or 9mm Makarov. However, there have been successful blowback designs for rounds as powerful as the 9mm Parabellum and the 9mm Largo.  These remain the exception, and not the rule.  In the small blowback operated self-loading pistol, the breech, or slide, is held against the cartridge head by a spring.  When the firearm is discharged, the bullet is pushed out of the muzzle with great rapidity, and at the same time, the case is pushing the slide back just fast enough to extract and eject the fired case after the pressure has fallen to zero.  If the slide were to open too rapidly, while there is still a high pressure situation in the barrel, the case will likely rupture, and powder gas will escape to the rear – a very unpleasant event!


Examples of blowback operated pistols include the Browning Models 1903, 1910, and 1922, the Mauser 1910, 1914, 1934, and HSc, the Walther PP and PPK, the Soviet Pistolet Makarova, the Czech Vz50 and Vz70, the  Astra Models 300, 400, and 600, and the Beretta Models 1915, 1934, and 1935.  So common is the blowback operating system that to name all the pistols that used it would take many many kilobytes to do so!


The blowback mechanism, while admirable for its simplicity, does not quite fit the bill for more powerful cartridges.  While it could be made to work, it would require pistols to have a) massive slides or breech mechanisms that would provide an unduly large and ungainly pistol, b) very powerful recoil springs that might make retraction or cocking of the mechanism difficult for those of smaller stature or strength, and c) very high quality of materials which, if longevity of the piece is desired, would make the firearm prohibitively expensive.

However, there are other principles besides weight which can be used to slow down, or delay a blow back action.  In a straight blowback action, the case pushes the slide or breech directly to the rear, so that the full thrust of the case’s rearward momentum is exerted to the rear.  Instead of using additional weight, the rearward motion of the slide or breech can be delayed by setting up the mechanism so that the thrust to the rear is operating at a mechanical disadvantage.  Or, as Hatcher puts it “So that it takes more push to produce the same amount of motion, or the same push to produce less motion.”


czright [2]



One example of this is the Czech Vz52 pistol.   In order for the slide to move to the rear under the impetus of the fired case, the rollers of the delay mechanism must first be forced from their recesses in the slide back into the roller mechanism – a process which requires the reward movement of the slide to exert force in a perpendicular plane prior to being able to move to the rear.  Because of this delay mechanism, the Vz52  is capable of using the powerful 7.62x25mm cartridge.

Another example of a delayed blowback mechanism can be found in the Remington Model 51 pistol.  In this design, the breech bolt is separate and not part of the slide.  When the cartridge is fired, the bolt recoils at high speed and strikes the slide.  Under this impact, the slide moves to the rear and lifts the breech bolt out of its locking recess in the frame.  This type of delay mechanism is sometime known as “impinging action” or “delay block” action.


For the use of more powerful cartridges, it is generally conceded that the barrel and breech must somehow be mechanically locked together during the period of high pressure.  In the short recoil method, the barrel and breech are locked together when the mechanism is in battery.  When the firearm is discharged the recoil drives both units (slide and and breech) to the rear at once.  However, after a brief interval of movement, significantly less than the length of a cartridge, the barrel is unlocked from the breech, and stops all rearward motion.  The breech continues to the rear, impelled by the momentum of the short period of locked travel and any residual gas pressure that may remain in the case.


This is the mechanism that is used in the vast majority of self-loading pistols chambered for the medium to powerful class of cartridges such as the 9mm Parabellum, .38 ACP, 9mm Largo, .38 Super, .40 Smith & Wesson, 10mm Automatic, and .45 ACP to name a few.

Star A [3]

Star A

In some cases, the slide is locked to the barrel by interlocking grooves on the top to the barrel and the roof or ejection port of the slide.  This is the method favored by Browning inspired designs such as the High Power, the Colt Government Model, the Star A, B, M, P, Super, the Glock, and the SIG Sauer designs.

In others, the slide and barrel are locked by means of a pivoting locking wedge on the barrel that engages locking surfaces on the slide.  In the locked position, the block is cammed up so that the barrel is locked to the slide.  In  the unlocked position, the locking block is cammed downward, freeing the slide.  This is the method used on the Walther P.38 and its derivatives such as the Beretta Model 92.

Beretta [4]



Still others, notably the Luger designs are locked by a knee-like toggle.  As long as the knee is pushed directly to the rear, the breech remains locked.  However, when an upward force is applied to joint of the knee or toggle, the toggle can break and rise vertically, allowing the breech block to move to the rear.  On firing the Luger, the barrel extension and breech block assembly move to the rear as a unit for a short distance.  Then, the toggle strikes the “ears” on the rear of the Luger’s frame which cams the “knee” upward and breaks the toggle.


In the long recoil mechanism, the barrel and breech are locked together for a distance greater to than the length of a cartridge.  At the rearmost extent of recoil travel, the barrel is freed to travel forward, and is then followed by the breech, which strips a new cartridge into the receiver.  Typically, in a pistol of this type, there are two return springs, one for the barrel, and one for the breech mechanism.

At the moment of firing, the breech and barrel are locked together.  They recoil to the rear a set distance (usually just longer than the length of a cartridge), until movement is arrested by a stop provided for that purpose.  At the rearmost position the breech mechanism is caught and held by a latch.   The barrel is then forced forward by its return spring, unlocking from the breech as it moves to the fore.  As the barrel goes forward, the empty case which is held to the breech by the extractor, is ejected.  When the barrel reaches its in battery position, it strikes a lever, which drops the latch and allows the breech to move forward.

Frommer Stop [5]

Frommer Stop

Not surprisingly, there were comparatively few successful pistols that used this method of operation.  The Hungarian Frommer Stop, is one such example, and functions quite well, despite the very complex mechanism.



In gas operated systems, some of the propellant gas from the fired cartridge is tapped off and used to reciprocate the pistol’s mechanism.  There have been very few pistols designed for this mechanism.  Usually, in a gas operating mechanism, there is a hole drilled in the barrel, and some of the gas from the explosion passes through the hole and acts on a piston, driving it to the rear with sufficient force to unlock and open the breech.



Self-loading pistols have been, and are, products that are limited only by the human mind and imagination.  Hopefully, this article has served to give you a rough idea of the basic operating mechanisms for the majority of self-loaders available.


Have you used a gun on a target range? What type of pistol did you use?

Monday, May 26 – Happy Memorial Day! – Founding RU member ADRIENNE GIORDANO  returns!


Bio: Adam Firestone brings more than 25 years of experience with weapon systems including small arms, artillery, armor, area denial systems and precision guided munitions to Romance University. Additionally, Adam is an accomplished small arms instructor, editor, literary consultant and co-author of a recently published work on the production of rifles in the United States for Allied forces during the First World War.

Adam has been providing general and technical editing services to authors and publishing houses specializing in firearms books since the early 2000s. Additionally, Adam provides literary consulting services to fiction authors including action scene choreography, technical vetting and technical editing. In this line of experience, Adam has had the fortune to work with well known authors including Shannon McKenna and Elizabeth Jennings.

Check out Adam’s blog here: http://adamfirestoneconsultant.blogspot.com/ [6]

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11 Comments (Open | Close)

11 Comments To "How “Automatic” Pistols Really Work by Adam Firestone"

#1 Comment By Becke Martin Davis On May 23, 2014 @ 9:22 am

Hi Adam – I remember taking a lot of notes when you talked about pistols at the OVRWA workshop awhile back. Now if I can just find the notebook I took all the notes in, I’ll be just fine. Since I moved, I’ve been driving myself crazy trying to find notebooks and papers I had filed away neatly in my old house.

Whenever I come across a scene with weapons – whether it’s in books, in a movie or on TV – I wonder what mistakes you’d catch.

I know you work with some authors – how does that process work? Do you help with staging the scene, or do you correct edits? (I don’t have anything ready for this, but I’m looking ahead!)

#2 Comment By Adam Firestone On May 24, 2014 @ 3:51 pm

Hi Becke,

With respect to working with authors, I do both…

All the best,


#3 Comment By Jennifer Tanner On May 23, 2014 @ 2:13 pm

Hi Adam…

This comment from Michael Cipolla was in my inbox this morning….

Like the technical analysis of what is needed and how used. One thing I wonder about is when the shooter retrieves the fired case. Since the charge will generate heat the case will be hot. Should the shooter be wearing gloves of some type or how long after firing will the case be cooled enough to pickup bare handed?

#4 Comment By Adam Firestone On May 24, 2014 @ 3:52 pm

Hi Jennifer!

Generally speaking, by the time the shooter gets to the case, it will be cool enough (though still warm) to handle easily.



#5 Comment By Carrie Spencer On May 23, 2014 @ 11:40 pm

Evening Adam…

I’ve read your article twice…=) I never KNEW all that went on inside! How often does something go wrong with the process? Are there steps you can take to minimize problems, like cleaning the gun more often or higher quality shells? (Sorry, hope those aren’t silly questions!)


#6 Comment By Adam Firestone On May 24, 2014 @ 3:55 pm

Hi Carrie,

Modern self-loading pistols are very reliable. I have (just because I was curious) fired literally thousands of rounds through many types of self-loading pistols without a failure.

That being said, using higher quality ammunition as well as cleaning and lubricating the firearm properly will always enhance reliability.

All the best,


#7 Comment By Douglas L. Marriott On April 30, 2015 @ 4:24 pm

I am a retired consulting engineer with a fair amount of experience in applied mechanics. I have been given a job by my company (retired? Don’t ask!)to unrvel the mysteries of semi-automatic reloading cycles. Though I taught mechanics for 25 years, my specific knowledge of gun mechanisms is shallow and not very long lived. My 1st gun is a 1916 Lee-Enfield Mk III, bought 2 years ago on my 74 th birthday. I find the “common wisdom” on semi-automatic actions neither common nor very wise, with Newton’s laws, momentum and shock waves bandied around with, I suspect, not much understanding of how they actually work. Your essay as a breath of fresh air. You do know what you are talking about. I would therefore like very much to get in direct contact with you to seek advice and references to technically sound literature on the subject. Would you be willing to have a phone conveersation with me? My number is 513-494-1110 Cell# 513-519-9370. I live in South Lebanon, Ohio 45065.
Doug Marriott

#8 Pingback By blowback | Lex maniac On August 1, 2015 @ 6:34 pm

[…] refers to a means of loading cartridges in a semi-automatic pistol or rifle. (See impassioned technical explanation here.) The idea seems to be that some of the gases under pressure generated in firing a bullet are […]

#9 Comment By sarfraz On March 24, 2016 @ 8:16 am

excellent literature

#10 Pingback By A basic ‘How does it work’ question – Glock Forum On May 19, 2016 @ 11:16 pm

[…] […]

#11 Comment By Strongarm On December 6, 2016 @ 2:07 am

Dear Adam,

Vz52 should not be considered as a delayed blowback handgun. It has a recoiling barrel to get the energy to unlock the slide from the barrel. From beginning to end of the barrel and slide together recoiling back, it is an absolute locked breech firearm. At separation stage, the rearward speed may be different for both by cause of the canted locking surfaces however. Delayed blowback guns need a breechbolt carrier and an accelerator to transmit the gained momentum resulting slower speed for actual recoil taking breechbolt. Heckler Koch P9 and Korriphila pistols are true delayed blowback samples.