Guide to Gunmetal

Guide to Gunmetal
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.

Read about different types of gunmetal and take a look at the author's comprehensive list of different types.

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.


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.

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 Gunmetals

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.

Recommended for You

The Marlin Model 1895-.444 Marlin is a handy, powerful rifle capable of taking down elk, moose, hogs, black bear and deer. Lever-Action

Marlin Model 1895-.444 Marlin

J. Scott Rupp

The Marlin Model 1895-.444 Marlin is a handy, powerful rifle capable of taking down elk,...

The Federal Berger Hybrid Hunter Ammo combines high BCs with a forgiving bullet profile and promises versatile performance. Ammo

Federal Berger Hybrid Hunter Ammo

Brad Fitzpatrick - April 30, 2019

The Federal Berger Hybrid Hunter Ammo combines high BCs with a forgiving bullet profile and...

Want to get into the long-range game and not go broke? Check out the Mossberg MVP Precision Rifle. Reviews

Review: Mossberg MVP Precision Rifle

J. Scott Rupp - March 21, 2019

Want to get into the long-range game and not go broke? Check out the Mossberg MVP Precision...

See More Recommendations

Popular Videos

Hornady 6MM Creedmoor

Hornady 6MM Creedmoor

Tom Beckstrand and Neal Emery of Hornady highlight the 6MM Creedmoor ammo.

Gun Clips with Joe Mantegna - BULLPUPS

Gun Clips with Joe Mantegna - BULLPUPS

Joe Mantegna talks about the origins of Bullpups.

Ruger Launches New American Rifle Predator in 6.5 Grendel

Ruger Launches New American Rifle Predator in 6.5 Grendel

OSG's Lynn Burkhead and Ruger's Matt WIlson kick off SHOT Show 2018 by taking a look at the Ruger Predator.

See more Popular Videos

Trending Stories

The handloading question: With large availability factory ammo on the market, why bother with reloading? Craig Boddington offers a few answers. Reloading

Reloading Ammo – Why?

Craig Boddington - March 26, 2019

The handloading question: With large availability factory ammo on the market, why bother with...

The Hi-Point 10mm carbine, technically the 1095 TS, sports a 17.5-inch barrel, is 32 inches long and weighs seven pounds empty. Semi-Auto

Review: Hi-Point 1095 TS 10mm Carbine

James Tarr - April 04, 2019

The Hi-Point 10mm carbine, technically the 1095 TS, sports a 17.5-inch barrel, is 32 inches...

Some history and reloading recipes on five popular .17-caliber cartridges, including the .17 Ackley Hornet, .17 Hornady Hornet, .17 Mach IV, .17 Remington Fireball and .17 Remington. Reloading

.17-Caliber Reloading Data and History for 5 Cartridges

Layne Simpson - June 05, 2019

Some history and reloading recipes on five popular .17-caliber cartridges, including the .17...

See More Stories

More Gunsmithing

Join Dick Metcalf as he visits the Ruger factory to put polymer and aluminum parts through tests to see which is stronger. Gunsmithing

Is Polymer Stronger Than Aluminum?

Brock Norman - May 31, 2011

Join Dick Metcalf as he visits the Ruger factory to put polymer and aluminum parts through...

          How to select the right components for your first DIY AR-15.    By David M. Fortier   Gunsmithing

The AR Build

David Fortier - December 30, 2010

How to select the right components for your first DIY AR-15. By David M. Fortier ...

Lapping protects the scope tube from damage; eliminates stress on the tube that could affect its integrity, and aids in improving accuracy Gunsmithing

How to Lap Scope Rings

Joseph von Benedikt - June 22, 2015

Lapping protects the scope tube from damage; eliminates stress on the tube that could affect...

See More Gunsmithing

GET THE MAGAZINE Subscribe & Save

Temporary Price Reduction.

SUBSCRIBE NOW

Give a Gift   |   Subscriber Services

PREVIEW THIS MONTH'S ISSUE

GET THE NEWSLETTER Join the List and Never Miss a Thing.