Rockwell Hardness & the Secret HRC Specs For the AK-47 Revealed!

We scoured the internet to find the original Russian Izhevsk factory blueprints and will be revealing the original Rockwell hardness specifications prescribed in these rare, hard-to-find, blueprints.

This article will also help to demystify some of the common practices involved in metal manufacturing.

So, if you’ve ever wondered what Rockwell hardness is, or how steel is forged, we break down some of the processes involved in the production of AKs, or firearms in general, and the machines used to test the hardness of various metals.

So, keep reading if you want to learn something new and are dying to witness the rarely-seen Soviet Avtomat Kalashnikov hardness specifications!

Note: We will also be updating this article with the hardness specs we collect on our own collection of modern AKMs (both import & domestic) as well as from old com bloc parts kits and de-milled receivers and barrels from days long past.

If you want to skip past the hardness Q&A and get to the old Soviet AK-47 Kalashnikov Avtomat HRC specs click here.

First, a few nuggets of knowledge to take up some real estate in your already hard-wired brain…

What Is Hardness?

Hardness, in general, is the ability of a material to resist deformation. Hardness is a characteristic of a material and not a fundamental physical property.

We all know that diamonds are the hardest materials on Earth and that’s why they are used in many industrial factories to cut materials that cannot be cut otherwise.

Diamond tips are also used in ultrasonic hardness testers as well as some indentation hardness testers to gauge the hardness of high-carbon metals of various thicknesses.

Why Is Hardness Testing Used?

1. Material Characteristics
   • Test to check material
   • Test hardenability
   • Test to confirm the process 
   • Can be used to predict Tensile strength

2. Functionality
• Test to confirm the ability to function as designed.
• Wear Resistance
• Toughness
• Resistance to impact

What Are The Rockwell Scales Of Hardness?

The Rockwell scale is a hardness scale based on the indentation hardness of a material. The Rockwell test measures the depth of penetration of an indenter under a large load (major load) compared to the penetration made by a preload (minor load).

There are different scales, denoted by a single letter, that use different loads or indenters. The result is a dimensionless number noted as HRA, HRB, HRC, etc., where the last letter is the respective Rockwell scale (see below). When testing metals, indentation hardness correlates linearly with tensile strength.

There are many more scales of hardness for many different materials that we have not included here as they are not relevant to the scope of this article.

Therefore, we will only be focusing on the HRC scale as this is the scale used for hard to very hard metals like those used in the manufacture of AKs.

What Is A Hardness Tester?

A hardness tester is a precision-built machine or device that is used to determine the hardness of materials.

It can be a permanent, floor-standing, machine or tabletop device, in most cases, but with recent advances in technology, some are now small enough to be portable.

What Are The Types Of Hardness Testers?

There are different types of hardness testers depending on the testing method in question. The first 3 are considered stationary testers that require a dedicated space to own and operate.

The Rockwell hardness testers come in various sizes and shapes but are mostly industrial in their size and application.

Since a majority of them require a strong amount of force to test the metal in question, they are very heavy as a result and cannot be used to test in the field.

Brinell testers are similar to Rockwell testers but can vary considerably depending on the type and size of metal objects they are designed to test.

Vickers & Knoop testers are desktop-sized devices with scopes and screens to view and measure the indentations left by the diamond.

The last two types of hardness testers are portable types and were designed to be used in the field or anywhere you so choose.

They have the advantage of testing non-destructively and can be used to test parts that larger testers can’t reach.

UCI or Ultrasonic hardness testers are small portable devices that come with a pre-programmed, handheld computational device, and a probe that contains the Vickers diamond.

Leeb hardness testers use a metal ball bearing in their probe that measures the rebound of the metal being tested to determine its hardness.

It can be used on metals with a more coarse or porous grain structure like cast iron and other cast metals.

Hardness Testing Methods

There are 4 methods of testing hardness and each has a unique name method of testing. Here are the 6 methods in question:

• Rockwell
• Brinell
• Vickers
• “Vickers Method” UCI
• Leeb
• Knoop

ROCKWELL:

The Rockwell hardness test method, as defined in ASTM E-18, is the most commonly used hardness test method.

The Rockwell test is generally easier to perform, and more accurate than other types of hardness testing methods.

The Rockwell test method is used on all metals, except in conditions where the test metal structure or surface conditions would introduce too many variations.

BRINELL:

The Brinell hardness test method used to determine Brinell hardness is defined in ASTM E10.

Most commonly it is used to test materials that have a structure that is too coarse or that have a surface that is too rough to be tested using another test method, e.g., castings and forgings.

VICKERS:

The Vickers hardness test method also referred to as a microhardness test method is mostly used for small parts, thin sections, or case depth work.

The Vickers method is based on an optical measurement system. The Microhardness test procedure, ASTM E-384, specifies a range of light loads using a diamond indenter to make an indentation which is measured and converted to a hardness value.

It is very useful for testing on a wide type of materials, but test samples must be highly polished to enable measuring the size of the impressions. A square base pyramid-shaped diamond is used for testing in the Vickers scale.

UCI (ULTRASONIC CONTACT IMPEDANCE) FOR HARNESS TESTING:

For this method, the test indentation size of the Vickers diamond is not evaluated optically but measured electronically when the corresponding test load is reached. This measurement process is performed in a fraction of a second.

Sometimes called modified Vickers Method UCI (Ultrasonic Contact Impedance) is standardized in ASTM A1038 or DIN 50159-1/-2 respectively and is also described in the guide VDI/VDE 2616.

Ultrasonic hardness testers display test results in Rockwell C, Brinell, and Vickers.

LEEB:

The Leeb Rebound Hardness Test invented by Swiss company Proceq SA is one of the four most used methods for testing metal hardness.

This portable method is mainly used for testing sufficiently large workpieces. It measures the coefficient of restitution. It is a form of nondestructive testing.

KNOOP:

Everything from the Vickers method applies except a pyramid-shaped diamond is used for testing in the Knoop scale. This indenter differs from the pyramid indenter used on a Vickers test.

The Knoop indenter is more elongated or rectangular in shape. The Knoop method is commonly used when indentations are closely spaced or very near the edge of the sample.

Note: For the purposes of this article, we will only be focusing on the Ultrasonic method of testing Rockwell hardness.

How Is Steel Forged?

Ever wonder how steel is forged? It’s quite a bit more complicated than you might have thought (ourselves included!).

The following information we gathered from Wikipedia gives us the answers to all of the methods involved in the steel forging process. It’s quite impressive!

Forging is a manufacturing process involving the shaping of metal using localized compressive forces. The blows are delivered with a hammer (often a power hammer) or a die.

Forging is often classified according to the temperature at which it is performed: cold forging (a type of cold working), warm forging, or hot forging (a type of hot working). For the latter two, the metal is heated, usually in a forge.

Source: Wikipedia

The following examples are some of the most commonly used ways to forge steel:

DROP FORGING:

Drop forging is a forging process where a hammer is raised and then “dropped” into the workpiece to deform it according to the shape of the die.

There are two types of drop forging: open-die drop forging and impression-die (or closed-die) drop forging. As the names imply, the difference is in the shape of the die, with the former not fully enclosing the workpiece, while the latter does. (Wikipedia)

OPEN-DIE DROP FORGING:

Open-die forging is also known as smith forging. In open-die forging, a hammer strikes and deforms the workpiece, which is placed on a stationary anvil.

Open-die forging gets its name from the fact that the dies (the surfaces that are in contact with the workpiece) do not enclose the workpiece, allowing it to flow except where contacted by the dies. (Wikipedia)

IMPRESSION-DIE FORGING:

Impression-die forging is also called “closed-die forging”. In impression-die forging, the metal is placed in a die resembling a mold, which is attached to an anvil.

Usually, the hammer die is shaped as well. The hammer is then dropped on the workpiece, causing the metal to flow and fill the die cavities. The hammer is generally in contact with the workpiece on the scale of milliseconds. (Wikipedia)

PRESS FORGING:

Press forging works by slowly applying a continuous pressure or force, which differs from the near-instantaneous impact of drop-hammer forging.

The amount of time the dies are in contact with the workpiece is measured in seconds (as compared to the milliseconds of drop-hammer forges). The press forging operation can be done either cold or hot. (Wikipedia)

UPSET FORGING:

Upset forging increases the diameter of the workpiece by compressing its length. Based on the number of pieces produced, this is the most widely used forging process.

A few examples of common parts produced using the upset forging process are engine valves, couplings, bolts, screws, and other fasteners. Upset forging is usually done in special high-speed machines called crank presses.

The machines are usually set up to work in the horizontal plane, to facilitate the quick exchange of workpieces from one station to the next, but upsetting can also be done in a vertical crank press or a hydraulic press. (Wikipedia)

ROLL FORGING:

Roll forging is a process where round or flat bar stock is reduced in thickness and increased in length. Roll forging is performed using two cylindrical or semi-cylindrical rolls, each containing one or more shaped grooves.

A heated bar is inserted into the rolls and when it hits a spot the rolls rotate and the bar is progressively shaped as it is rolled through the machine. The piece is then transferred to the next set of grooves or turned around and reinserted into the same grooves.

This continues until the desired shape and size are achieved. The advantage of this process is there is no flash and it imparts a favorable grain structure into the workpiece. (Wikipedia)

COLD FORGING:

Near net shape forging is most common when parts are forged without heating the slug, bar, or billet. Aluminum is a common material that can be cold forged depending on the final shape. Lubrication of the parts being formed is critical to increasing the life of the mating dies. (Wikipedia)

Note: Cold hammer-forging is the most common method for ensuring durability in barrels, trunnions, bolts, carriers, triggers, hammers, receivers, etc.

Forging Of Steel

Depending on the forming temperature steel forging can be divided into:

• Hot forging of steel
  • Forging temperatures above the recrystallization temperature between 950–1250 °C
  • Good formability
  • Low forming forces
  • Constant tensile strength of the workpieces

• Warm forging of steel
  • Forging temperatures between 750–950 °C
  • Less or no scaling at the workpiece surface
  • Narrower tolerances achievable than in hot forging
  • Limited formability and higher forming forces than for hot forging
  • Lower forming forces than in cold forming

• Cold forging of steel
  • Forging temperatures at room conditions, self-heating up to 150 °C due to the forming energy
  • Narrowest tolerances achievable
  • No scaling at the workpiece surface
  • Increase of strength and decrease of ductility due to strain hardening
  • Low formability and high forming forces are necessary

For industrial processes, steel alloys are primarily forged in hot conditions. Brass, bronze, copper, precious metals, and their alloys are manufactured by cold forging processes, while each metal requires a different forging temperature. (Wikipedia)

What Steels Are Used To Make AK Rifles?

All older AK and modern AK parts, as well as most other firearms, are made using a mixture of *Chrome-Moly, or Ordnance-Grade, steels, specifically 4130, 4140, and 4150.

Trunnions, barrels, bolts, carriers, fire control groups, gas blocks, sight posts, and receivers are all made from one of the aforementioned low-alloy steels. 4150 is common in barrels, due to its higher carbon content. 4140 is used for

*Exceptions: Some US-made AKs from Century Arms use billet-milled S7 “shock-resistant” tool steel to make certain parts- like the trunnions, bolt carrier, and feed ramp.

Also, AKs are not all made from cold-hammer-forged parts. Some of the less vital parts are cast like the rear trunnion, carrier, sight posts, fire control groups, etc- though some AKs do have hammer-forged carriers as well.

It’s more important to have CHF parts where strength & durability is needed most: the bolt, front trunnion, and barrel.

What Is 41XX Chrome-Moly?

Most people from my generation (Gen X) can remember BMX bikes like Mongoose had chrome-moly frames that made them lightweight and strong but did you know that chrome-moly is also used a lot in the making of firearms?

41xx steel is a family of SAE steel grades, as specified by the Society of Automotive Engineers (SAE). Alloying elements include chromium and molybdenum, and as a result these materials are often informally referred to as chromoly steel (common variant stylings include chrome-moly, cro-moly, CrMo, CRMO, CR-MOLY, and similar).

They have an excellent strength to weight ratio and are considerably stronger and harder than standard 1020 steel, but are not easily welded, requiring thermal treatment both before and after welding to avoid cold cracking.

While these grades of steel do contain chromium, it is not in great enough quantities to provide the corrosion resistance found in stainless steel.

Examples of applications for 4130, 4140 and 4145 include structural tubing, bicycle frames, gas bottles for transportation of pressurized gases, firearm parts, clutch and flywheel components, and roll cages.

4150 stands out as being one of the steels accepted for use in M16 rifle and M4 carbine barrels by the United States military. These steels are also used in aircraft parts and therefore 41xx grade structural tubing is sometimes referred to as “aircraft tubing”.

Source: Wikipedia

What Steels Are Barrels Made From?

By and large, most barrels are made of Chrome-Moly steels like 4150/4140, followed by Stainless Steels and other (more exotic and lightweight) materials like Columbium, Aluminum, Titanium, Carbon Fiber, Fiberglass, Metal–Matrix Composites, Ceramics, and Metal Alloys engineered at the atomic particle level.

Steel makers offer special “ordnance-grade” steel alloys for the manufacture of gun barrels. Ordnance-grade alloys are subjected to special handling, careful heat treatment and rigorous analysis to assure quality and consistency.

The majority of barrels are monolithic—that is, the entire barrel is made out of a single piece of metal. Here, it should be noted that different rifling methods utilized by barrel makers (more on that in a bit) often work best with specific steel alloys and levels of hardness.

Most barrel steels have a hardness of around 25 to 32 on the Rockwell C scale. Suitable metal alloys for monolithic gun barrels include:

nrafamily.org

ALLOY STEELS:

Suitable barrel alloys include types having varying amounts of chromium, molybdenum, vanadium, nickel, and manganese as alloying metals.

A popular example and perennial favorite in the U.S. is called SAE 4140 chrome-molybdenum or “chrome-moly” steel. In Europe, vanadium and nickel-steel alloys are preferred.

Most standard barrels are made with alloy steels, as they offer an excellent balance of strength and cost. (nrafamily.org)

The Hardness Specs For The Kalashnikov Avtomat AK-47

And now, what you’ve all been waiting for… drum roll, please!

We spent some odd hours scouring through dozens of original AK-47 and AKM blueprints to find all of the hardness specifications for every part that makes up a finished AK and are publishing these findings for the first time ever on our blog- Exoc Tactical.

If these hardness specifications were available anywhere online- in their absolute completeness- we couldn’t find them! Trust me when I say we spent many days looking to no avail.

No, we can’t read Russian (though we could use Google Lens to translate) but it wasn’t necessary to find the HRC specs since they’re labeled as such in the blueprints. So here they are:

AK-47 & AKM SOVIET HRC SPECIFICATIONS:

Skipped to the good stuff and want to go back to what you missed? Click here.

If you found this article useful, feel free to share it anywhere (with a link and credit to its author). We will be publishing and comparing the hardness specs on as many modern AKMs as we can get our hands on in the coming months so check back to see what we find out!

How will the specs compare to the original Kalashnikov? Who’s making parts up to spec and who’s not? How do older parts kits from other Com-Bloc countries compare? All will be revealed in good time…