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AR-15 Bolt Action Rifles

Guide to Gun Metal

by Patrick Sweeney   |  December 29th, 2011 38
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.
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