Schneier on Security

Clive Robinson • January 21, 2012 4:12 AM

For those not up on military weapons a little background information.

About 100years ago soldiers were armed with rifles of various forms that were designed to be accurate out to 500meters as would be requiired for infantry style fighting in open country against other infantry or “horse” Cavalry who were usually armed with some kind of sword. Around this time senior NCO’s had hand weapons such as a revolver and officers were armed with what was essentialy a bit of wood with metal end caps.

The first world war changed all of this, fighting was nolonger short actions in open countryside. It was prolonged in “dug in positions” and urban environments such as villages and towns and the attrition rate on men and materials was very high.

Thus the rifle aimed at shooting at ranks of soldiers at 300meters or more was compleatly mismatched to the type of warfare. The machine gun became the standard for trench warfare in that you could lay down “fields of fire” acuratly thus alowing your own troops to advance under cover of heavy fire aimed at the enemy trenches.

However the machine gun has two distinct differences over a magazine rifle due to rate of fire and they are important to know.

The first issue with a high rate of fire is heat, due to various issues each time you fire a gun the breach and the barrel heat up quickly and cool down slowly. The result is that under a sustained high rate of fire the barrel can get sufficiently hot that the metal lands of the rifleing “goes soft” and it’s rate of wear can be measured in hours. A secodry issue with heat is that the breach can get so hot that when a round is chambered it can “cook off” and fire without the trigger being pressed. Which is a significant issue as most infantry rifles are “closed breach” weapons as this gives greater accuracy when firing from the shoulder and also reduces the amount of stopages due to dirt and mud etc that an infantry soldier crawles through on ttheir belly when fighting up to an enemy position.

A machine gun however has a very heavy sometimes water cooled barrel to deal with the heat and would be almost impossible to fire from the shoulder or in the prone position without significant support. Also as a covering weapon it does not have the same need of accuracy and is also usually mounted up high to get as an extended field of cover as possible. Thus it is no where near as susceptible to dirt, and can thus be an “open breach” weapon which tends to avoid the “cook off” problem.

The second issue with a high rate of fire is getting the rounds fed into the breach. Traditional magazines use springs to ensure a certain degree of positive preasure so that the rifle bolt will correctly scoop the round of the top of the magazine and chamber it without misfeeding. There is only a certain number of rounds a spring can deal withh effectivly and the limit for rifle rounds being fed upwards from a straight magazine is usually around 20-30 (modern materials science has improved this a bit). One of the first solutions was to turn the breach the other way up and use gravity to assist and you see this in weapons like the Bren gun where the gunner (who carried it) had an assistant whose job it was to carry and replace the top fed magazines when it was being fired. These straight feed down magazines also had a limit due partly to the spring but a lot to do with the shape of the amunition (rifle amunition used to be rimed like revolver amunition to aid in manual force ejection of a spent round) and it was usually between 30-50 rounds.

The solution with a machine gun is to realise that you can use part of the energy of firing a round to eject the round and further to realise that you could further use that energy to replace the spring in a magazine by using a “film projector motion”. The problem with this is you need quite a heavy duty spring to store the energy required for chambering the next round and this means that the breach design likewise has to be heavy.

Thus the development of machine guns and rifles folowed very different paths for quite some time.

However there was and still is a need for a high rate of fire for covering a patrol as they advance on an enemy position, hence a third style of weapon was needed that had the range and accuracy of a rifle but a high rate of fire thus the “patrol rifle” or “light support weapon”.

However the infantry of today has a problem due to the increase in urban or close quaters fighting a long barreled weapon is a definate disadvantage so various designs such as the “bull pup” have moved the breach down the stock of the rifle so it is very close to the shoulder (and as any soldier who has fired the SA80 or similar left handed will know the hot brass burns down the inside of your shirt due to this) or have shortened the weapon.

Whilst improving the infantry mans ability to fight at close quaters the patrol or light support weapon has not shortend. Thus as infantry soldiers will tell you being the holder of the patrol weapon or long barrel sniper weapon makes you an easily “marked man” for an enemy sniper.

Which is why there are now designs for infantry weapons to be personal light support weapons which are bullpup and importantly open breach whilst also using special light high temprature low wear alloys for the breach block and barrel. The downside of course is the high tempratures they can now reach stand out realy realy well on an “IR heat scope” but as everybody in the patrol gets one including the PL or OC they are all equaly as marked which means you don’t want to be the point man…