The story of steel
TH EST EELS USED for barrels, with t h eir incorporated chambers and some of the components in the actions, are expecte d to e ndure extreme mechanical stresses. The purity and correct formulation of the alloys and an appropriate heat treatment are crucial in achieving the necessary strength and durability. However, one reads, usually on the internet, of catastrophic failures in guns, of which some may be attributed to faulty steel.
In one instance, an allegedly flawed batch of 10 ML-II stainless steel possibly caused the catastrophic destruction of a firearm, a serious injury to the shooter and a subsequent lawsuit. But most of these reported failures remain unverified by a competent investigation. Fortunately they are rare. To establish the facts, a microscopic analysis of the crystal structure of the metal at and near the fracture has to be done.
Interestingly, most metallurgical failures in guns are a consequence of poor heat treatment. I have seen this in a chipped firing pin; the small damage to the nose was due to brittle metal. On the other hand, bulged barrels and distortions in the action are usually the result of excessive stress from pressure, not faulty metal. The problem is that in the absence of a proper technical analysis, harmful hearsays can spread through our shooting community and especially on the internet. The following are examples of some truths and fallacies about gun metallurgy.
THE SOFT MAUSER: A belief prevails that the small-ring Mauser actions, namely Models 91, 93, 94, 95 and 96, which preceded the popular Model 98,
are weak. This is attributed to the supposedly inferior steel used in the rifles of Argentina, Sweden, Spain, Chile and Turkey. It is a myth. Firstly, all the 93 models were built in Germany by Mauser, Ludwig Loewe and DWM using German steel. In the case of the Swedish 96 Mauser, Swedish-made steel was of exceptional quality – a high grade base alloyed with nickel, copper, and vanadium. To this day it is noted for its exceptional strength and corrosion resistance. I have been unable to find any reports of Mauser actions failing due to poor quality metal. Of course the M96 action is not as robust as the M98 that followed it, but is a little shorter and lighter and has some pleasing features. It is certainly strong enough, and elegantly designed for the military calibres it was intended for, and the action serves well for many of the .30-calibres. Husqvarna produced an excellent rifle in .30-06 on this action.
What then was the origin of this belief? Surprisingly, it was due to an unfortunate direction taken by the Sporting Arms and Ammunition Manufacturers’ Institute (SAAMI) in the United States. Undue concern was raised in 1926 about the large number of small-ring Mauser rifles being imported into the USA with the prospect of their supposedly weak actions being re-used in sporting rifles chambered for various civilian medium calibres. This weakness was presumed due to comparison with the mechanically stronger M98 action, and this half-truth gave birth to the myth. Consequently, many cartridges that were expected to be used in these recycled actions were required by SAAMI to have low maximum average pressures (MAP). For example, the military 7x57mm cartridge designed for the supposedly weak Spanish Mauser was loaded for a MAP not exceeding 50 370psi, and it is unlikely that Spanish Mausers in military service had a
habit of exploding at this pressure. However, in the USA, SAAMI reduced this to 46 000psi. Both measures used the copper units of pressure (CUP) method. For the 6.5x55mm Swedish cartridge, SAAMI chose 51 000psi while the original military cartridge was specified for 55 110psi. This is a considerable difference but, readers, please note, this observation does not sanction ever going beyond the limits published in load tables. However, it does explain why some European-made ammunition is more potent than its American counterpart. For the technically minded, the MAP used by SAAMI is the 4% standard deviation from the average pressure measured in a test of five shots.
THE BURSTING ENFIELD: In 1987, a report by the British Ministry of Defence followed an investigation of two incidents where the chambers of Enfield No 4 rifles had fractured. It was found that extensive crazed cracking in the chambers was the cause. Crazed cracking is a myriad network of small interconnected cracks. It’s a metallurgical issue. Understandably, no extended investigation followed, as No 4 rifles were no longer in service and were mostly used for cadet drill. Also, the occurrences in just two retired rifles of which over a million had been made had to be considered. The conclusion was an instruction that if No 4 rifles were to be used for shooting, they should be inspected for metal integrity, especially in the chamber and bore. This instruction was prudent and reasonable. In our shooting sports this is the norm for any veteran firearm.
Unfortunately, an article followed in the American Rifleman, advising Enfield No 4 users to have their rifles checked by a gunsmith. Good advice – but for the following. I quote (the italics are mine): “If there are any signs of roughness or erosion in the barrel immediately ahead of the chamber, or any other visible defects in the barrel or chamber walls, then the barrel should be regarded as suspect and the rifle should not be fired until it has been properly fitted with a new barrel.” This is a misleading remark. Erosion had nothing to do with the reported failures. Erosion is often seen in retired British .303 rifles, especially considering that they may have fired thousands of rounds. The condition is due to the cordite propellant used in the service cartridges. It had a high burning temperature which left its mark in the lead into the rifling after a lifetime of use. The damage appears as a misty, satin-like surface just ahead of the chamber at the start of the bore. Such erosion does not occur in the chamber nor does it cause a weakness there. It is vexing that a single misuse of a word can bring the safety of countless numbers of these fine old rifles into question.
THE SHATTERING M1903 SPRINGFIELD: A few early M1903 Springfield rifles suffered chamber fractures which were attributed to burnt steel. General Hatcher, author of Hatcher’s Notebook, discovered that on one occasion, large bundles of bar-stock destined for barrels was heated to a forging temperature which was too high. The hot-forming was done to upset one end of the bar to a larger diameter for the chamber end of the barrel to have sufficient material for the machine-profiling that followed. Burnt steel is a condition where the crystalline structure, which gives the steel its strength, is compromised. Such occurrences are hardly significant considering the scale of production of the Springfield rifle. Metallurgical deficiencies were usually revealed by the compulsory five-shot proof test using ‘blue pill’ cartridges, so named for their blue colour indicating a 50% over-charge. Much like its contemporary European and British bolt-action war horses, the M1903 Springfield is a reliable rifle of excellent design.
THE CRACKED M16: Some failures in the bolts of M16 rifles were investigated by the US Military Academy’s Department of Civil and Mechanical Engineering. Their enviable access to binocular
microscopes, scanning electron microscopes and computer modelling resources revealed that the fracture initiated at a localized corrosion pit and was then propagated by mechanical fatigue. The localised deep corrosion pit was caused by imperfect surface heat treatment of the steel in that specific area and possibly neglected care of the gun. In addition to this, the proximity of a sharp notch in the design of the bolt may have contributed to the advance of the crack. Shooters should note that pitting in high stress areas is potentially dangerous. Although not strictly a metallurgical issue, the additional problem of sharp features such as the notch described in the report is worth explaining.
It is useful to imagine lines of force in a mechanical component under physical strain. The distribution of the stresses can be represented by straight lines in a smooth bar. However, when encountering the many corners as in the complexities of a rifle bolt, they will tend to converge at corners much like F1 cars do when taking the best line through the apex of a sharp corner. This clustering of force lines creates a weak spot of concentrated stress. Where possible, engineers relieve such a stress point by designing a gentle radius to partially relieve it.
Interestingly, most metallurgical failures in guns are a consequence of poor heat treatment
EARLY GUN STEELS AND THEIR WEAKNESS: Antique firearm barrels were forged from iron or cast in bronze or brass and then cold-formed to the final shape by primitive machining and filing. They had to be thick-walled in relation to the bore diameter because the metal was usually weakened by micro pockets of gas and inclusions of foreign material. This is why collectors seldom risk firing early antique pieces. Prior to the 20th century, most shotgun barrels were formed by heating wire or strips of iron and steel together while winding and beating them around a mandrel. The resulting weld fused it into a more-or-less homogeneous metal tube. This was Damascus steel. An etching and polishing process revealed the attractive figure in the metal. Old Damascus barrels are always suspect and certainly inadequate for nitro loads. There are some contradictions to this remark on the internet but they are wrong. A non-destructive determination is all but impossible to do and visual inspection is insufficient. However, some may be considered safe for a prescribed black powder load if they are Belgian or British barrels bearing proof marks.
At the same time, steel metallurgy was rapidly advancing. Two processes followed the ancient method of melting iron in a crucible. A system called the Bessemer process blew air through the iron smelt to burn off impurities, and later, the open hearth furnace using oxygen followed. To this day it is sometimes used as the first step before the electric or vacuum furnaces to make the hi-tech alloys in our guns. Barrel strength runs parallel to these developments. The Snider Enfield Mk III rifles were made of rather primitive steel with very low carbon content. The breech block was forged from rolled billet material, machined and then possibly heat-treated by quenching in oil to provide some surface hardening. Martini Henry rifles had superior steel, most likely made in the Bessemer process. It was alloyed with some nickel and a little silicon. It is possible that some of the components were given a case-hardening treatment in molten salt.
Scientific studies of catastrophic firearm failures, most of which were done at military facilities, show that very few can be attributed to metallurgical problems. Most are caused by obstructions in the bore such as a cleaning patch or a bore-sight bushing. But less frequent and more surprising causes have been reported such as oil or grease on the cartridge or a soft cartridge base. Some were recounted by General Hatcher in his meticulously kept notes. The most common were ballistic faults such as a shot following a lodged bullet in the bore.
By far the most such failures occur in the civilian world through incorrect charges or wrong bullets in handloaded cartridges. This is some food for thought.