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AR-15 Bolt Action Rifles

Guide to Gun Metal

by Patrick Sweeney   |  December 29th, 2011 30
rifle receivers

Rifle receivers, which have to be very strong, also require a great deal of machining. It’s not easy to pick a steel that will serve as a receiver but not wear out cutting tools too quickly.

Gun metal terms get bandied about in product literature and the firearms press as if everybody knew just what the hell they were talking about. If you’re in the dark about what it all means, read on.

What is steel? And why is it so important in gun building? Simply put, steel is iron with enough carbon in it to allow hardening—but not too much because that makes the resulting alloy brittle. Steel does not have pores; it consists of crystals. (Brief rant: Had I any hair left, I’d be pulling it out every time I heard of yet another lubricant that “gets into the pores of the steel.”) The shape, size and alignment of those crystals determine the mechanical properties of the steel in question. The crystals of steel are described by their sizes and shapes, and they have actual names such as austenite and martensite, cementite and ferrite.

Steel can be alloyed with other metals such as nickel, chromium and tungsten—as well as non-metallic elements as molybdenum, sulfur and silicon. Those alloying agents add useful things to the mix, such as easy machineability, corrosion resistance, abrasion resistance or tensile strength without brittleness to the steel grade in question.

The Society of Automotive Engineers uses a simple designating system, the four numbers you see bandied about in gun articles. Numbers such as 1060, 4140 or 5150 all designate how much of what is in them.

The first number is what class—carbon, nickel, chromium and so forth. The next three numbers tell you how much of what is in them. Let’s take as an example the steels in the classic barrel argument amongst AR owners: 4140 steel versus 4150 steel.

4140, also known as ordnance steel,  was one of the early high-alloy steels, used in 1920s’ aircraft frames and automotive axles in addition to rifle barrels. It has about 1 percent chromium, 0.25 percent molybdenum, 0.4 percent carbon, 1 percent manganese, around 0.2 percent silicon and no more than 0.035 percent phosphorus and no more than 0.04 percent sulphur. That leaves most of it, 94.25 percent, iron.

trigger guard

Something like a trigger guard doesn’t have to be made of high-strength alloy steel. Mild steel, easy to machine and relatively inexpensive, works just fine.

The “big” difference between 4140 and 4150? The 4150 has 0.5 percent carbon in it. That extra 0.1 percent makes the 4150 alloy so much harder that it becomes a lot more difficult to work with, but the U.S. Army wants the extra wearability that 4150 offers and is willing to pay for it.

Most rifle makers realize that their customers won’t pay the extra costs and find that 4140 is more than good enough. After all, if a .30-06 hunting rifle already has a barrel that will shoot accurately for 5,000 rounds—which is like three lifetimes of hunting—who will pay twice the barrel cost for one that lasts 7,500 rounds?

However, the SAE standards are merely a list of ingredients. When, and at what temperatures you add the alloying constituents also can change the final properties. AR-15 bolts, for instance, are made of a steel known as Carpenter 158. It is a product of the Carpenter steel company, the sole maker, and you won’t find it on the SAE list (although if you did it would probably be known as 3310). It’s Carpenter’s secret, proprietary steel, and if you want it, you buy it from Carpenter.

Are there steels that would work as well, or even better, than Carpenter 158 for AR bolts? Probably. The alloy is a product of 1960s technology, and we’ve learned a lot since then, but it is enshrined as the mil-spec.

And what about stainless steel? Developed before World War I, stainless steel used in firearms isn’t really stainless. It is very rust-resistant, however—not because there is so much chromium but because the chromium on the surface reacts to air to form a passive layer of chromium oxide, which seals the iron from oxidation.

Stainless alloys have their own designations, and the most common of these are the 400 series, and 416 is very popular with manufacturers because it is almost as easy to machine as carbon steel.

Aluminum is used in firearms in two alloys: 7075 and 6061. 6061 is commonly referred to as “aircraft” aluminum and has trace amounts of silicon, copper, manganese, molybdenum and zinc. 7075 is a much stronger alloy and has markedly larger amounts of copper, manganese, chromium and zinc.

rifle bolts

Forged bolts are really, really strong, And tough to machine. Some firms still make them as one piece, but many manufacturers have figured a way to make strong bolts of two pieces that are welded together.

Either are strong enough for the tasks we ask of them, but the big reason for 7075 over 6061 in the production of AR receivers, for instance, is corrosion resistance. Early testing in Southeast Asia showed that human sweat, combined with the high temperatures and humidity of the jungle, would simply eat away at 6061 alloy. 7075 just shrugs it off.

Aluminum is too soft to be used bare. To harden its surface, manufacturers use a process known as anodizing. They dunk the aluminum parts in a tank of an acidic solution and pump electricity through it. The result is an accelerated formation of natural oxides that harden the surface.

The oxides are porous, so it is common to use a sealant. The mil-spec process uses a nickel acetate sealant, and the dark color results from the dye used (the natural color left after anodizing is still “aluminum”).

What does this mean for us rifle shooters? Well, now you have a better idea of what gun companies (and gun magazines) are talking about when they spew metal specs at you when describing a gun’s construction.

Common Gun Metals

Carbon Steels

  • 1020 and 1520—Common, “plain” or cold-rolled steel. You’ll find it in trigger guards, floorplates, sights, sling swivels and other steel hardware.
  • 4140—Ordnance steel or chrome-moly steel, it has 0.4 percent carbon and is really strong while still being cost-effective to machine. You’ll find this in barrels, bolts receivers and high-stress items like muzzle brakes.
  • 4150—The same as “ordnance” steel but with the carbon content upped to 0.5 percent. 4150 holds up better to serious abuse, and it’s found primarily in mil-spec AR-15 barrels.
  • 41V45—A chrome-moly variant, it has a dash of vanadium in it. This is an alloy selected to produce hammer-forged barrels.
  • 8620—This is a full-up alloy of nickel, chromium, molybdenum, with 0.2 percent carbon. Cast receivers are made of this alloy because it fills the mold well, machines cleanly and ends up very tough and strong.

Stainless Steels

  • 316—Also known as “marine” grade stainless, as it resists corrosion well because of added molybdenum but is not easy to harden. Used in trigger guards and floorplates.
  • 17-4—An alloy with 17 percent chromium and 4 percent nickel. 17-4 (or a close kin) is readily hardened and is used in barrels, bolts and receivers.

Aluminum Alloys

  • 6061—Aircraft aluminum, selected in that application for its light weight and ease of fabrication into complex parts. Floorplates on hunting rifles, scope rings and some handguards and buffer tubes on AR-15 rifles are made of 6061.
  • 7075—Much stronger than 6061, it’s the alloy used in AR-15 upper and lower receivers, some mil-spec brands of buffer tubes and some railed handguards. In mil-spec parlance, it is known as “7057-T6”; the last part designates the type of heat treatment it receives.
  • Jake Dorsey

    Very informative, and quite fascinating, actually! Thank you for writing this. Knowing bits about metals used in knives never did seem to help with gun metal specifications. It's interesting the two don't seem to overlap (at least in this article).

  • Mack Missiletoe

    So how does 41V45 compare to 4150 steel?

    • arizonajake

      Mechanical properties are very similar between the 2 alloys. The Vanadium in 41V45 makes the steel a little easier to cold-form, which is how hammer forging of barrels is done.

      • Mack Missiletoe

        Thanks!

  • Firebird

    Excellent and usefull information. Thank you very much.

  • arizonajake

    As a metallurgical engineer, I foud this article to be very informative and accurate. Your fine publication would do all of us a great favor by starting to debunk claims by lubricant peddlers that stress their producs "penetrate the pores of the metal…." THIS IS NOT TRUE!!! Porous metal in firearms is defective material. Properly machined surfaces are solid and smooth and lubricants attach to these surfaces by surface tension and viscosity, NOT by "penetrating into the pores of the metal". It just drives me up the wall to hear people parrot this myth. People's skin has pores. Properly machined surfaces requiring lubrication do not. Thanks for letting me rant and keep up the good work!

    • coldhard4150

      This industrial engineer agrees. It's basic materials science/engineering. I kept waiting for this article to state something incorrectly, or miss the point altogether, but it's dead nuts on.

  • KMK678GC

    i found your article to be comprehensive and most informative; thank you

  • Dyspeptic Gunsmith

    1. The most common stainless steels in industry are the 300-series steels, particularly 304 stainless. The 300-series stainless steels weld better than the 400's, and they're more corrosion resistant, in general. 416 stainless is used in firearms because it is easier to machine due to the addition of sulphur. As the author correctly points out, "stainless" is not corrosion proof – in fact, it is now possible to blue stainless steel barrels. If it can be blued, then it is being corroded, by definition. Some stainless receivers, slides, etc are made from "17-4PH" where the "PH" stands for "precipitation hardening."

    2. 4150 isn't that much more difficult to machine than 4140 or 4130, especially for a shop using modern carbide insert tooling. With modern CNC machines, you can select your inserts to your material properties and away you go.

    4140 is simply much more common and it is highly versatile. If I had to choose only one type/grade of steel to have in my shop for everything, it would be 4140. It machines well, welds well, takes heat treatment well with an easy protocol. Second to that would be 4340, but it is much more difficult to heat treat and weld correctly. Properly heat treated, 4340 can be up to three times as strong as 4140.

    3. 8620 has been used for a lot of gun receivers over the years. It takes a case hardening well, and case hardening leaves a core in the casting/forging/piece that is softer and tough, but the outer case prevents dents, scratches and wear. The higher carbon content steels will harden clear through if left in the heat treatment oven long enough, which one might not want in a rifle receiver application.

    4. Last, but not least, one should NB that with the evisceration of our steel and manufacturing industries by the pointy-headed academics running our government, much of the steel you get in the market today is from off-shore and does NOT meet AISI/SAE specs for content or strength and the quality is crap. Chinese steel in particular is a crap. It doesn't matter whether you specify 4140 steel. It matters that you actually HAVE 4140 steel. Gun manufactures will use "certified" 4140 for many of their critical parts (bolts, receivers, etc), and "gun barrel quality" steel for the barrels, which is even higher quality. "Gun barrel quality" steel is difficult to obtain, there are only three steel dealers I know of where I can obtain it.

    • http://www.facebook.com/people/Jane-Snape/100001971991197 Jane Snape

      Excellent rundown of metal types.

      As for your point #4: I haven’t seen any “pointy-headed academics running our government”. I have seen a lot of bought-and-paid-for politicians who loosened the laws that kept robber-baron “captains of industry” from running off to China, Vietnam or other places that provide slave labor. It’s no coincidence that the richest 1%’s wealth has nearly tripled since 1979, even as the rest of us have been lucky not to fall behind.

      • dubbs

        Don’t,DARE tell that truth! Teabag voters and extreme rifhtwingers blame,the “lazy dumb american worker” for not wanting to be a slave laborer like in china or mexico!

        The major corporations blame the workers for poor quality , but never admit to cutting costs and using shoddy materials to keep prices down while making a profit for the SAME 1-5% of the population!

        Our shame is that our great nation is buying steel and metals from recycled amerian metal which has been reprocessed overseas by lower standards having producers.

        Sooner or later it will make its way into our weapons ised to defend this nation( I was,at Camp Pendleton MCRD on a civie training course, and was disgusted to see boxes for products like tables and other equip with “made in china” marked on them…..)

        • Frank Scott

          Amen, again!

      • Frank Scott

        Amen, brother. And we need to get the politicians to use a little strategic lawmaking to bring it back. Let the Chinese make their own steel, kiddie toys and shoes.

  • Mack Missiletoe

    Man, we gun people need to know this stuff!

  • http://twitter.com/houndsnmules @houndsnmules

    great article, Please do another about steels and harding.

  • bhp9

    Much of the expert testimony here is pure baloney. Metal is porous and oil, water and corrosive salts do indeed penetrate metal. Here are some real life examples of the porosity of cheap cast metals that are used in many modern made firearms today.

    Castings soak up both oil and water. Not keeping a cheap casting well oiled will result in a very rapid onset of deep penetrating rust that will eat a hole right through the side wall of a thick rifle receiver. I have seen this happen with cheap cast receivers. This does not happen with the older forged receivers found on older classic guns, yes they get surface rust and if left neglected the rust does eat away at the surface of the metal but it is not the devastating deep penetration you see with cheap porous castings.

    Bullet molds will also soak up water or oil. Any one who has ever oiled even a lukewarm mold after casting bullets with it soon finds out that even most solvents will not remove the soaked in oil. The mold must be boiled in detergent soap for at least 1/2 hour or the mold will seep oil out of it when it is warmed up again.

    • STBro

      There can be significant differences in steel both at the surface and in the bulk material caused by variations in the production of the bulk metal, mechanical processes used to form the metal, thermal processes applied, chemical processes, etc. Additionally, the alloy type, service conditions, and environmental conditions can change the surface quality of the metal over time. So, it is possible to have steel that is free of detectable pores even when viewed with a Scanning Electron Microscope (SEM). It is also possible to have steel with pores that can be detected by inspecting the sample with a simple magnifying glass and a flashlight. For contained pores, radiography is generally employed. In most cases, porosity is considered a discontinuity and if sufficiently severe, a defect. Thus, a porous steel sample would be rejected if porosity exceeded acceptance standards (for example ASTM A802 or B765.) However, there are alloys specifically engineered and produced to be porous. Steel can also exhibit degradation such as pitting that is similar to porosity. This is always a defect. I have seen finished products that passed basic visual tests but would weep drops of water when subjected to high pressure hydrostatic test. Dye penetrant testing revealed porosity that went all the way through the metal. These were defective. Have I seen firearms with porous metal? Yes. Would I use those firearms? I did. It is a question of where the porosity occurs and what the required standard is for that part. I have also seen high quality firearms that exhibited no detectable porosity.

  • bhp9

    Corrosive ammo will penetrate metal pores in warm barrels and or microscopic cracks in barrels and the salts will seep out for weeks on end, even after cleaning said barrel. This is why corrosive ammo often destroys so many barrels even though they have been previously cleaned. The German army for years cleaned their barrels rigorously 3 days running to try and prevent this seepage from annihilating their barrels.

    If you have a cheap cast modern made gun keep it well soaked in oil and grease or risk the rapid and vigorous on set of deep penetrating rust and the pitting that results from it. The Germans found out years ago that a good forging made properly actually resisted rust very well, before stainless steel began to be used in the making of firearms. The Germans went to great pains to make a durable forging for their South American contract Mauser rifles. Yes they did eventually rust in the humid air of South American but they held up for years as opposed to the rapid deterioration of porous castings when used in firearms.

  • bhp9

    Have you noticed that more than one custom barrel maker does not lap their button rifled chrome-moly barrels but they do lap their stainless barrels. What does that tell you when their reputation rides on rifle barrel accuracy. It means that the stainless barrel is a lot more difficult to make shoot accurately because of the difficulty in rifling stainless barrels with their naturally sticky metal. You do not get as smooth a rifling so hand lapping is a must when making the stainless barrel as opposed to the chrome-moly. Stainless barrels do not last any longer nor are they any more accurate either.

    All this is old news as the great Gunsmith P.O. Ackley proved this decades ago. The only advantage in stainless was more rust resistance but they rust and pit too, if severely abused or neglected which is the norm with most gun owners.

  • Pavan

    It is common for shotgun manufacturers in India to make barrels with "steam pipe". They vouch by its suitability and durability. All shotgun manufacturers in India are legally required to get their barrels tested by Indian Ordinance Factory, and these "steam pipe" barrels seem to have no difficulty in passing such tests. I often wondered what grade steel this "steam-pipe" thing is? On the other hand, handful of shotgun manufacturers in India use EN19, and claim that it is superiror to the "steam pipe". I googled a bit about EN19, and found it to have a HRC of around 50 after hardening and tempering. I also read that usually shotgun barrels have a HRC between 28-32 HRC, and harder steels usually result in brittleness of the barrel. If it is so, then I am confused as to how EN19 would be superior to the "steam pipe", Could you please throw some light on what should be the steel for a shotgun, and what is this "steam pipe steel".

    • coldhard4150

      50 really isn't that brittle. There is an inverse (give and take) relationship between hardness and ductility. A Rockwell Hardness of 80 would be too brittle for use in a rifle barrel. 50 gives the rifle a good amount of durability through hardness. The annealing and quenching process is a factor which lets the manufacturer choose the hardness of the metal.

  • bob r

    anyone, does a cold rolled shotgun barrel have enough strenth to safely use modern shot gun 12ga birdshot

  • Markku P

    You write "Either are strong enough for the tasks we ask of them, but the big reason for 7075 over 6061 in the production of AR receivers, for instance, is corrosion resistance. Early testing in Southeast Asia showed that human sweat, combined with the high temperatures and humidity of the jungle, would simply eat away at 6061 alloy. 7075 just shrugs it off."

    That is contradictory to what other sources tell about corrosion resistance between 7075 and 6061, there is a lot of information about the subject in the Internet. 6061 is in general considered more corrosion resistant. There are, of course, several variants of the both 7075 and 6061. Also much depends on the anodizing method. Is there some type of corrosive strain that is better resisted by 7075? Is sweat an exception of the general rule? As far as corrosion is concerned, 6061 is considered better in salty sea climate, for example.

    • dubbs

      But 6061 grade aluminum is softer and easier to damage and warp, making it technically INFERIOR to 7075 T6 grade aluminum.

      It amazes me that when cointing cost and use, people still want to go cheaper materials then claim they are “just as good”.

      O have older weapons with most likely 4140 grade barrels, and I have several ARs with ORD 4150 and CMV barrels. I’d rather depend on my higher grade steel barreled rifles….

  • ilyo

    just for curiosity, which is more harder, surgical (dental/surgery stainless steel) or firearm steel? excluding the barrel.

    thnx in advance.

    ilyo

  • Suburban

    7075 aluminum is pretty commonly used for aircraft parts. It is, therefore, also proper to call 7075 “aircraft aluminum.”

  • Jon Allie

    Here’s an idea…. coat the metal gun parts with a graphene ink and or use a graphene metal alloy to make some of the parts. And how about the bullets…..
    Can you make sintered bullets using a bismuth 93% tin 6% 1%
    graphene powder then coat with a graphene ink to include a permination of the
    surface for strength? This would be idea since it would mimic the performance
    of LEAD Alloy bullets without the toxicity and might also maintain a good
    penetration and hold together rather than to become a frangible round so might
    improve and lessen mfg costs for making combat ammo.
    http://store.vorbeck.com/pages/bulk-orders Graphene Ink for Bullets Call today
    for a quote: 301-497-9000 or email us at sales@vorbeck.com Use less
    expensive metals with graphene coating which improves penetration and prevents
    oxidation

    • ANON

      Better rifle parts and better bullets – Molybdenum improves the tensile strength and wear resistance of Liquid Phase Sintered Alloys (Fe-Mo-B), and increases their hardenability. The addition of boron in iron-molybdenum alloys activates the sintering process and increases the alloys density. In addition, small amounts of Graphene & DU Powder can be added to create better hard surface piecing high velocity bullets. This involves minor changes to the mix in patent http://www.google.com/patents/US5950064. Also, this would work best for both Devel and THV bullets – refer to web site http://www.firearmstactical.com/tacticalbriefs/volume4/number2/article422.htm

  • max

    Wanting a 50 BMG but am not wealthy. Forged crankshaft out of a 5.7 350 engine. Can it be machined and turned into a action/receiver?

  • max

    Oh ya liked article.

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