Blowback, Direct Impingement Gas, Short & Long-Stroke PISTON

When a semi-auto or fully-automatic firearm cycles - ejecting the fired case and chambering the next round there are many different systems designed to do the job.

There are also many different considerations that effect the design - such as weight, size, reliability, complexity to manufacture and cost. - It's not a simple question of "what is best?" for all operating conditions and requirements.

The excellent moving pictures and much of this information are from Wikipedia - my usual first port of call when researching information for these shooting articles.

The ' Blow-Back' system where the rear face of the cartridge sits against a heavy  bolt and slide assembly that is driven back directly by the rearwards force of the shell-case (pushed by the expanding gasses) is a simple design that depends on the inertia of the bolt and spring to hold the breech shut until the bullet has left the barrel - it works well if weight is not critical and the cartridge is 'low-powered' such as a pistol round - eg. 9MM or .22" long-rifle.

Simple Blow-Back System Operation.

 

The fixed barrel blow-back operating system has been used for pistol calibre guns and sub-machine guns and is most suitable and very frequently used for .22" rim-fire rifles and pistols. The down-side of blow-backs is that it can leave sooty residues on the operating surfaces exposed to the escaping gas and may require frequent cleaning. The well known excellent Ruger 10/22  is an example of blow-back operation.

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The Direct Impingement System, such as that used in the AR15 rifles uses gas bled-off from the rifle barrel and directed back down a tube to directly push the bolt and carrier rearwards and to cycle the re-loading sequence.

AR15 / M16 Direct Gas Impingement System

 

This uses the minimum number of parts moving back & forth to disturb the rifle - but does direct the hot gasses and powder residues straight into the rifle mechanism causing rapid increases in temperature and leaves dirty gummy fouling on the action while burning-off lubrication and reducing service life.

Early Pattern M16.

 

This system has been rudely described as "pooping where you eat" - and does require regular cleaning and use of special high-tech, high-temperature lubricant.

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Short-Stroke Piston operating system uses the gas bled-off from a barrel to drive a short-stroke piston to the rear that then hits a tappet or the bolt carrier directly imparting the rearward impulse while itself stopping. This is the SKS and M1 Carbine system.

 

 

This short hard push transfers kinetic energy to the bolt carrier assembly by 'hitting it' while keeping the dirty hot gasses away from the mechanism.

 

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With a Long-Stroke Piston System - the piston is fixed to the bolt carrier system and moves with it throughout the whole ejection / reloading cycle.

 

 

This is the operating design of the Kalashnikov AK47, the Bren Gun and others. While keeping the hot gasses well away from the mechanisms - the added mass or weight of the piston / carrier gives increased impetus and reliability to the guns - but that same extra weight does have some effect on the guns accuracy by disrupting the centre of mass more - and does "stop & start" more noticeably and requires extra gas to be bled-off to operate the longer stroke.

    Classic Image of an AK47

 

Everyone has  their own likes and dislikes depending on their personal priorities - ultimately it all comes down to what feels best in your hands, shoots well enough - and what you can afford to buy/use.

 - There are other less common operating systems such as electric motor and pneumatic, - delayed blow-back etc. but as I don't have any direct experience of them I'll pass ..

 

Marty K