For some 500 years it’s been known that rifling would impart spin and therefore stabilization to a ball or bullet. Spiral grooves probably evolved from straight grooves only intended to trap powder fouling; by 1500 gunsmiths in Augsburg, Germany, were rifling their arquebuses. This gave rise to an early attempt at gun control, according to W.S. Curtis in Long Range Shooting, An Historical Perspective:
In the early 16th Century there are references to banning grooved barrels because they were unfair. Students of the duel will recognize this problem arising three hundred years later.
— Curtis, 2001. Curtis notes that why rifling was twisted is unknown, and that it may have been incompletely understood. He has quite a few interesting historical references, including one to a philosopher who explained that if you spun the ball fast enough, the demon (who dwelt in gunpowder, which was surely Satan’s own substance) couldn’t stay on and guide your ball astray. (Curtis’s work is worth beginning at the beginning, which is here).
By the mid-19th Century, the Newtonian physics of the rifled bore had been sorted out, the Minié and similar balls made rifled muskets as quick-loading as smoothbores, and the scientific method allowed engineers to test hypotheses systematically by experimentation. So smoothbores were gone for quite a while (they would return in the 20th Century in pursuit of extreme velocities, as in tank guns).
Rifling had several effects beyond greater accuracy. It did decrease muzzle velocity slightly, and it did increase waste heat in the barrel. The first of these was no big deal, and the latter was easily handled, at first, by improved metallurgy. But rifling also helps retain highly corrosive combustion by-products in the bore; and corrosion was extremely damaging to rifling. Pitted rifling itself might not have too much of an effect on accuracy (surprisingly), but the fouling that collected in the pits did. Corrosion also weakened the material of barrels, but most military barrels had such great reserves of strength that this was immaterial, also.
Fouling and pitting have been the bête noire of rifles from 1498 in Augsburg to, frankly, today. A badly pitted barrel can only be restored by relining the barrel, a job for a skilled gunsmith with, at least, first-class measuring tools and a precision lathe with a long bed. Relining has never been accepted, to the best of our knowledge, by any military worldwide.
Chrome Plating is Invented: 1911-1924
One approach has been to use corrosion-resistant materials for barrels, but that has been late in coming (late 20th Century) because it is, of course, metallurgy-dependent. Early in the 20th Century, though, American scientists and engineers developed a new technology — electroplating. George Sargent, of UNH and Cornell, worked with chromium as early as 1911, and Columbia scientists developed a commercially practical process of using electrodes to deposit chromium by 1924. Meanwhile a New Jersey professor worked with a German process.
The two groups of professors formed start-ups, the Chemical Treatment Company and the Chromium Products Corporation. At this point, chrome plating has not been applied to firearms. Electroplating had been used for guns for decades, of course, but that was nickel plating — eye-pleasing, but soft and prone to flaking, not suitable for bores, and not remotely as corrosion-resistant as chromium.
(This article is rather long, so it is continued after the #More link below. We next take up the application of this process to rifle bores).
Chrome comes to bores in the lab: 1925-32
One thing that had held chrome plating back was lack of a practical quality control method. George Dubpernell discovered a practical test almost by accident: chrome would adhere to copper, but copper would not adhere to chrome. This was later supplanted by NDT methods, but it was essential to the growth of chrome in industry.
Olin’s and Schuricht’s patent of 1932 (not 1935, a rare error in Emerson), US Patent 1,886,218, applied chrome plating to small arms and sporting weapons’ bores. They applied for the patent in 1927, and note, as is now well known, that bores must be made slightly oversized to account for the dimensional changes from chrome deposition. They also, interestingly, saw chrome plating as a way to restore worn rifling and eroded barrels. We’re unaware of any such use being brought into practice in the intervening decades.
Meanwhile, in 1937, T.K. Vincent noted that:
Chromium plating of small arms barrels results in longer accuracy life. However, the cost of plating is excessive compared to the results obtained.
The longer accuracy life results from taming the bugbear of bore erosion. By 1942, in a thorough study of bore erosion of guns large and small (from 3″ naval guns to small arms), Burlew noted a report by Russell that considered chrome plate a “bad” material from a bore-erosion standpoint, except “when made very adherent”; in that case it was an “excellent” material, roughly five to nine times better than ordinary plating. Chrome-plated steel barely edged out bare steel, and all beat exotic metals like Inconel and Monel; the least erosion was found in the chrome-plated barrels with the thinnest chrome plating (0.0005″), although all these tests were of a 12″ naval gun, and their applicability to small arms might not be direct or proportional.
The technology of chrome plating continued to advance, even as weapons designers struggled to bring the technology’s benefits to bear on practical small arms.
Adoption of chrome by the world’s militaries — early adopters
The Empire of Japan was the earliest nation to chrome the bores of its rifles. The Japanese had different reasons, perhaps, than other nations. In Japan, supply of high-quality steel was insufficient to wartime requirements. This is especially true after 1940, when the United States imposed sanctions on the island nation, which depended on imports for almost all resouces; and even more true as unrestricted submarine warfare, which was ordered implemented even as the Pearl Harbor strike force was recovering on their carriers, began to strangle the home islands.
Casting about for a way to work with the second-rate steels they had, the engineers at Sagami Arsenal, which was used for ammunition storage and for war production (Japan’s only 100-ton tank was built here; it was too heavy to move to the seaport for deployment) set upon a 1937 patent. They concluded that chrome-plated mild steel could substitute for some high-speed and high-carbon steels, and from 1940 that’s what Japanese engineers did. The history of a Japanese firm explains:
The Japan Science Council reported then Government to recommend the policy to apply hard chrome plating on the low grade steel as the alternative to high grade one, such as special steel or high-speed steel, under the difficult external trade conditions to get them, the invention, Patent No.131175 (1937), “the method to deposit hard and thick metal chrome plating” by Minoru Araki, the former president of Company, being as the technical foundation. It was followed by the request to establish a specialized company of hard chrome plating (industrial chrome plating) from National Headquarters of Aviation, Sagami Arsenal, and customers.
As a result, the next rifle adopted by Japan, the Type 99 Arisaka 7.7mm rifle, had a chrome-plated bore. As David Petzal writes for Field and Stream, they were “the first military barrels ever to have this feature.”
The industrial and materials-science reasoning behind Japanese chroming is missing from most US sources. Gordon Rottman (a fellow SF veteran) writes that , “the Japanese had the foresight to produce the type 99 with a chrome-plated board to prolong barrel life, ease cleaning, and protect it from tropical rust.”
In addition to the Type 99s, all of which were intended to be made with chrome-lined bores, all Type 100 submachine guns, some late Type 38 6.5mm Arisakas, and some late Type 14 “Nambu” pistols had chrome-lined bores. By late in the war, ever more serious materials shortages meant that chrome bores were one of the features deleted from late production guns (like such Type 99 features as a monopod).
The United States initially chromed only large-caliber artillery bores. From Navweaps.com:
In the 1930s, the USN started to chrome plate the bores of most guns to a depth of 0.0005 inches (0.013 mm). This was “hard chrome,” which is not the kind that you find on your father’s Oldsmobile. This plating increased barrel life by as much as 25%. The plating generally extended over the length of the rifling and shot seating. Chrome plating has also been found to reduce copper deposits.
All along, as a large body of scientific papers at DTIC reveals, US small arms developers continued to work on chrome for small arms. US engineers were aided in this by their very great extent to which chrome was being used in the automotive industry. Springfield Armory developers would have had access to many papers being produced at the same time by the SAE, and Springfield of course worked closely with the developers, themselves, of chrome industrial processes.
But chrome was not standardized for US small arms bores until after World War II — in fact, not until the mid-1950s, well after Japanese and Russian adoption of the technology. As we’ve recounted here before, the first US weapon to be manufactured new with a chrome bore was the M14 rifle. Around the same time, chrome bores were used in developing a 7.62 mm NATO conversion kit for the Browning light machine guns, and replacement barrels that were manufactured for Legacy weapons like the M1 rifle, started to be manufactured with chrome bores as well.
Because chrome bores lost some definition in the rifling, and therefore some accuracy, National Match rifles continue to be produced with standard bores. But the advantages of chrome in the field could not be overlooked.
The M16 rifle was initially produced without a chrome bore. There are two reasons for this: first, the M16 was a product of a private industry initiative, and not the usual Army development system. The disastrous fielding of the M-16, with the bare bore combined with very poor maintenance practices and some units, led to the Army adding a chrome chamber, and then finally a chrome bore to the weapon.
Another assembly of the M-16 was chromed, and this led to a lot of problems. The part in question was the entire bolt carrier group. Early on, a number of the bolts and bolt carriers failed. This turned out to be due to metallurgical problems, specifically with heat treating (that will sound familiar to anyone who has followed the M14 history), the deficiencies of which were masked by the plating, and also with hydrogen embrittlement of the steel carrier during the chroming process. The specification was changed to require the bolt to be Parkerized, except for its internal expansion chamber, and the inside of the bolt carrier key, which are still chromed (chroming only a single surface of a part does not risk hydrogen embrittlement).
Early chrome BCGs that were properly heat-treated and passed testing were allowed to remain in M16A1s by the Army, but they were not allowed to be deployed OCONUS. The reason given (in the M16 maintenance manual, TM9-1005-319-23&P) is simply to prevent glare off a chrome bolt carrier from exposing soldiers’ positions.
The USSR‘s reasons for introducing chrome plating (whether for corrosion control, ease of cleaning, or metallurgy) are unknown to us, but extensive collector interest makes it clear when the feature was added: 1950. No known 1949 SKS or AK rifles have chrome bores, some 1950 models do, and almost all 1951 and subsequent guns do. Chinese AK and SKS rifles were produced with chrome bores from their introduction in 1956. Some satellites’ bores were not chromed, notably Yugoslavia’s pre-1970s. (Yugoslavia was technically not a “satellite,” but it was a Eurasian communist country).
For practical purposes, this means that all Soviet and Chinese spec AKs will have chrome bores. In addition, gas pistons are also chromed. This greatly facilitates cleaning, and prevents corrosion in a highly corrosion-prone part of the system.
Russian small arms of larger caliber, including the 37mm tube of the RPG-7V, are also chromed.
Adoption of chrome by the world’s militaries – later adopters
Belgium, a small country that looms large in world firearms exports thanks to FN, was not an early adopter of chrome bores. The entire production of the FN-49, including all ABL, SAFN, and AFN rifles, left the FN factory with conventional steel bores. Much later, metric pattern FALs received, first, chrome chambers, and later chrome bores. What makes FN interesting enough to comment on here is their use of chrome extended to the internal parts of their MGs and the insides of their receivers, making MAGs and Minimis very easy to clean.
US variants of these FN guns don’t have these parts chromed. The initial MAGs and Minimis purchased using using special funding vehicles by select US special operations units, had these features. In subsequent US production, the chroming was eliminated, and those parts of the M240 and M249 are Parkerized. We don’t know if this was done to save money, because the Army simply preferred the Parkerized coating, or because of the Army’s bad experience with chromed bolts on the M16A1.
Britain adopted chrome bores well after World War II, including some retrofits like the L4 Bren Gun from at least the L4A4 version to the final L4A9. As noted above, Britain’s inch-pattern FALs did not receive chrome bores.
Chrome chamber vs Chrome bore
Industrially speaking, each of these had its own pros and cons. Chroming the whole barrel was more expensive, increased demands for both manufacturing and inspection precision, required the rifling to be cut slightly oversize (to allow for the chromium deposition), and led to much greater waste. Chroming the chamber was a compromise that enhanced extraction — a sticky problem with many automatic arms — without the costs and problems associated with full-length bore chroming.
But the US experience showed that half a loaf (chroming the chamber only) didn’t get the job done. While the chamber became very resistant to corrosion, GI’s inspection of the bore often stopped with a glance in the chamber area, and if the chamber was gleaming, they’d assume the rifle was good to go — eveb as combustion byproducts and deposits ate away at the rifling.
Meanwhile, chrome bores let the manufacturers do things that were difficult or even impossible with conventional manufacturing processes. As noted above, the Japanese were able to use chromium plating to substitute for lack of chromoly steel. In the USA, Springfield Armory discovered that by slowly withdrawing the barrel, chamber first, from the chromium bath they could create a squeeze-bore effect due to the higher deposition of chrome on the parts of the barrel that were in the chrome bath longer. (Methods of altering the depth of chrome depositions produced at least two patents, 2,425,349 and 2,687,591; the second is Springfield’s process).
Chrome’s cost rises
In the 1970s, the chost of chromium suddenly went through the roof: the two greatest producers, Rhodesia and the USSR (ironically, two defunct nations, today) were locked out of the US market, the former by sanctions and the latter by international politics. (Note that around 1974 the styles of American cars began to use less chrome plate and more body-colored and black molding. This fashion was driven in part by costs).
Today, the biggest driver of rising plating costs is new environmental regulations. Chromium, like most metals, is something you really don’t want to breathe in.
Quality chrome plating is still expensive, and cheap plating produces a lot of waste. Some gun parts makers have chosen to, essentially, ignore the waste and ship products with poor (or zero!) nondestructive testing and inspection, sacrificial sample examination, or other valid QC.
Chrome plating today & tomorrow
Plating has to fight to maintain its place vis-a-vis other anticorrosion technologies, including noncorrosive metals (i.e. stainless steel) and superior steel coatings like Melonite, but it has a very strong position as an erosion fighter, particularly in barrels subject to high temperatures (think automatic fire).
Some scientists are working on electroplating as a means of additive manufacturing. Laugh if you like, but the plating industry of today was entirely based upon laboratory discoveries.
And gun engineers continue to apply new kinds of chromium treatment to bores. A recent patent application by Rheinmettal covers depositing a different thickness of chrome in the lands and the grooves of a rifled barrel.
One of the biggest changes is that a chrome-plated bore, if made with sufficient care, may be as accurate or more accurate than a bare bore. (For example, SAK manufacture M16 replacement barrels seem to outshoot many target barrels). But this may not be as big a change as you think. According to Emerson, in 1962 Springfield Armory made a small quantity of chromed National Match barrels. They discontinued the practice not because the barrels were bad, but because they were much more expensive to make than bare barrels, and they were not any better. But they were atdid fully comply with national match standards at the time.
Chrome-lined barrels are currently the standard in military small arms. This will change if and when something better comes down the pike – and not before.
Burlew, John S. The Erosion of Guns, Part One: Fundamentals of Ordnance Relating to Gun Erosion. Report No. A-90 Progress Report. Washington: National Defense Research Committee, 8 Sep 42. Retrieved from: http://www.dtic.mil/dtic/tr/fulltext/u2/a422462.pdf
Burlew, John S. The Erosion of Guns, Part Two: The Characteristics of Gun Erosion. Report No. A-91 Progress Report. Washington: National Defense Research Committee, 31 Oct 42. Retrieved from: http://www.dtic.mil/dtic/tr/fulltext/u2/b280242.pdf
Curtis, W.S. Long Range Shooting, An Historical Perspective. Research Press, 2001. Retrieved from: http://www.researchpress.co.uk/longrange/lrhistory.htm
Dubpernell, George. History of Chromium Plating. Products Finishing magazine, 13 Nov 12. Reprint of Plating & Surface Finishing from 1984. Retrieved from: http://www.pfonline.com/articles/history-of-chromium-plating
Emerson, Lee. M14 Rifle History and Development. Online Edition, 2007.
GlobalSecurity.org. Sagami Depot, Japan. n.d. Retrieved from: http://www.globalsecurity.org/military/facility/sagami-depot.htm
Koka Chrome Industry Ltd., Company History. n.d. (2011 or later). Retrieved from: http://www.koka-chrome.co.jp/en/company/history.html
Olin, John, and Schuricht, Alfons. Gun barrel and process of finishing the same. Washington, 1932: US Patent No. 1,886,218. Retrieved from: http://www.google.com/patents/US1886218.
Rottman, Gordon. Japanese Army in World War II: the South Pacific and New Guinea, 1942–43. 2005: Osprey Publishing. (p. 36).
US Army, Technical Manual: Unit and Direct Support Maintenance Manual (Including Repair Parts and Special Tools List): Rifle, 5.56mm M16A2; Carbine, 5.56mm M4; Carbine, 5.56mm M4A1. Washington, DC, 9 Apr 97
Vincent, T.K. Development of Chrome Plating of Guns. Abstract only (have been unable to find the full text). Aberdeen Proving Ground: Ballistics Research Labs, 1937. Retrieved from: http://oai.dtic.mil/oai/oai?&verb=getRecord&metadataPrefix=html&identifier=AD0701179