This work is an honest and reasonable attempt at capturing the history and development of the M14 rifle. The reader is encouraged to check the facts for himself. Topics such as development of the M1 Garand into the M14, gunsmithing procedures, match conditioning, stock refinishing, hand loading ammunition, rifle marksmanship, and close order drill have been purposefully omitted. Discussion of optics for the M14 rifle is only touched on lightly. Those subjects have been addressed at length by others much more capable.
Thank you to members of the www.ambackforum.com and www.m14forum.com Internet discussion boards for their support and helpful suggestions. Special thanks go to Sadlak Industries, Smith Enterprise, SparrowHawk Stocks, Troy Industries, Warbird's Custom Guns and William J. Ricca Surplus for their generous assistance.
The author thanks those who have contributed to this work but wish to remain anonymous. Other Sources # 12, # 18 and # 27 made significant contributions to this volume. The copyediting chore was freely and graciously donated by Brent A. Blanchard, Attorney and Counselor at Law. Mr. Blanchard learned to shoot from his dad as a child. He shot smallbore rifle as a teenager and was a member of the Brigham Young University Army ROTC Rifle Team. He competed in High Power competition matches in the 1980s while the M14 rifle was king of competition shooting. After he learned that careers in public relations, marketing, and financial planning were not to his liking, Mr. Blanchard went to law school. He was admitted to the bar in the State of Nevada after graduation and later accepted a position at a law firm in Las Vegas, NV.
Two literary customs are dispensed with in this work. There are no double quotes before and after specific alphanumeric characters used to identify equipment and rifle parts. Additionally, a space is intentionally placed between numerals and units of measure. The substantial number of rifle part identifying marks and dimensional measurements included in the narrow focus of this volume renders those conventions bothersome.
Due to the nature of the Internet, web site addresses listed in the Bibliography were active at the time research was conducted but may not work thereafter. Always handle all firearms in a safe manner. Consult with law enforcement officials or an attorney if you are unsure of the law where you live.
Lee Emerson December, 2004 Las Vegas, NV
The Military M14 Introduction The U.S. Rifle 7.62 mm M14 was adopted for military service by the United States on May 1, 1957. The M14 rifle was developed to replace four military firearms, M1 Garand rifle, M1 Carbine, M1918 Browning Automatic Rifle and the M3A1 submachine gun. The M in M14 stands for Model. The M14 is a rotating bolt, gas operated, air cooled, magazine fed, shoulder fired weapon. The M14 is 44.28 “ long with the hinged butt plate and weighs 8.7 pounds. With a full magazine, cleaning kit and sling it weighs 11 ¼ pounds approximately. The maximum effective range is 460 meters (503 yards). The M14 has seen hostile service with the American military from the 1963 Cuban missile crisis to the Second Gulf War. The M14 rifle has been employed as a battle rifle, squad automatic weapon, competition match rifle, grenade launcher, sniper rifle and ceremonial rifle.
Between 1958 and 1963, the U. S. government ordered 1,380,358 M14 rifles from four entities. These were the U. S. Army’s Springfield Armory in Springfield, MA; Winchester (Olin Mathieson Chemical Corp.) in New Haven, CT; Harrington & Richardson Arms Co. in Worcester, MA; and Thompson-Ramo-Wooldridge, Inc. in Cleveland, OH. A total of 1,376,031 M14 rifles were delivered between 1959 and 1964.
Engineering Background Information
Some basic background information is presented here to assist the reader later on. This will be helpful in understanding the manufacturing processes of M14 type rifle receivers and parts. There are three important factors which influence the quality of the M14 type rifle receiver and parts. These factors are material, heat treatment, and dimensional geometry.
If the proper material is not used, the specified (and desired) values for each mechanical property (hardness, strength, toughness, etc.) may not be achieved for a given part, regardless of whether heat treatment is performed well or done at all. If the incorrect material is used to make the part, the part may yield a shorter service life, or may even catastrophically fail with resulting personal injury or death. The same goes when heat treatment of parts is not performed according to design specifications and procedures. Whether a receiver is initially formed by forging, machining or casting, the finish machining operations are performed before carburizing, quenching and tempering. If dimensional geometry is incorrect, even a properly heat-treated part made of the correct material will either function poorly, it at all, or it will not last as long as it should.
The following terms are defined for this discussion:
A1 temperature – This is the minimum threshold temperature to create austenite molecular structure in steel. The A1 temperature is 1341 degrees Fahrenheit for steel with less than 6.67 % carbon content. All of the steel used in the M14 rifle contains less than 1.00 % carbon.
A3 temperature – The temperature at which ferrite forms as the steel begins cooling. This is about 1528 degrees Fahrenheit for AISI 8620 steel.
Alloy steel – This is steel with trace percentages of other elements.
Annealing - This is a method of heat treatment performed by heating steel to a temperature that transforms all of the molecules to austenite structure, followed by slow furnace cooling. This method produces a coarse pearlite molecular structure in the steel. Annealing is done to produce lower strength and higher ductility steel—performance characteristics opposite of high strength and brittleness.
Austenite – Austenite is the Face Centered Cubic molecular structure of iron. The maximum solubility of carbon in austenite is 2.11 %.
Carburizing – Carburizing is a group of techniques for heat treating the surface of steel. It is used when the steel alloy has insufficient carbon to attain the desired surface properties through conventional heating methods. Carburizing alters the chemistry of the surface of the steel. The first part of the process is the diffusion of carbon into the part’s surface at an elevated temperature. This creates a high carbon content at the surface which increases the hardness. When the part is then rapidly cooled and tempered the surface remains hard and strong while the core (or center) remains softer and tougher.
Case depth – The depth below the surface of a steel part to which hardening occurs by surface heat treatment techniques.
Cementite – This is a hard and brittle compound of three iron atoms and one carbon atom. It contains 6.67 % carbon. It is used to strengthen steel when it is dispersed evenly. The chemical formula is Fe3C.
Ferrite – Ferrite is the Body Centered Cubic form of iron. The maximum solubility of carbon in (alpha) ferrite is 0.0218 %. Alpha ferrite is one structure of several that can form upon slow cooling from the molten phase.
Hardness – Hardness can be thought of as resistance to permanent indentation. Hardness is measured using various tests with their own number scales, such as Brinell, Knoop, Rockwell, and Vickers.
Heat treatment – Heat treatment is the controlled heating and cooling of metals for the purpose of changing their physical properties. It is one of many methods that can be used to change the mechanical properties of metals.
Hypoeutectoid steel – This is steel with less than 0.77 % carbon content. If hypoeutectoid alloy steel is heated above the A1 temperature, 100 % austenite structure can be formed. This is desired for making hardened steel parts by heat treatment.
Magnetic particle inspection – Magnetic particle inspection is a method of non-destructive examination that detects surface and subsurface flaws in ferromagnetic metals (iron, steel, nickel and cobalt alloys).
Martensite - The Body Centered Tetragonal molecular structure of steel with 0.2 % or greater carbon content. It is the hardest, strongest and most brittle molecular structure of steel. It can be as hard as 65 HRC.
Ms – The temperature at which steel begins to form martensite molecules upon rapid cooling. This temperature is different for each steel alloy.
M90 – The temperature at which steel is composed of 90 % martensite upon rapid cooling. This temperature is different for each steel alloy.
Nitrocarburizing – Nitrocarburizing is a thermochemical process that improves the surface properties of iron or steel parts. It increases surface hardness to 60 HRC, reduces the coefficient of friction, improves wear resistance and enhances corrosion resistance of the metal treated.
Normalizing – This is a method of heat treatment that is performed by heating steel to a temperature that transforms all of the molecules to austenite structure followed by air cooling or oil quenching and tempering. This treatment produces a fine pearlite molecular structure in steel. Normalizing is done to control dispersion-strengthening of the steel.
Pearlite – A form of steel that contains two solid molecular structures of steel, ferrite and cementite. It is created when steel is slowly cooled.
Rockwell hardness test – This hardness tester uses a small diameter steel ball or diamond cone depending on the material to be sampled. The depth of penetration of the ball or cone is automatically measured and converted to a Rockwell hardness number, expressed with “HRC” or “HRD” after the number. HRC means the hardness value on the Rockwell C scale. HRD means the hardness value on the Rockwell D scale. The Rockwell C and D scales are used to measure the hardness of high strength steels. A diamond cone indenter is used for these hardness scales.
Stress – Applied force divided by the original material cross-section area.
Tempering – Tempering is the heating of martensite steel below the A1 or eutectoid temperature. This heating redistributes the cementite within the martensite. Redistribution or dispersion of the cementite is called dispersion-strengthening. Tempering relieves residual stresses caused by the transformation of austenite into martensite upon rapid cooling. Tempering reduces the strength and hardness but increases the ductility and toughness of steel. However, the hardness is also dependent upon the carbon content of the steel. The higher the carbon content the higher the hardness of the martensite.
Tensile strength – The strength of a material can be determined by measuring the stress it takes to cause deformation. The yield strength is the stress needed to begin permanent deformation (elongation) of a material. The ultimate tensile strength is the maximum applied stress before the material breaks.
Toughness – Mathematically, it is often measured as the total area under the stress versus strain curve for a given material. In layman’s terms, toughness is the resistance of a material to failure by impact.
Effects of Elements in AISI 4100 and 8600 Series Alloy Steels
In the United States, types of steel are commonly identified by the American Iron and Steel Institute (AISI) classification system. Carbon and alloy steels are given unique four digit numbers. The first digit of each number indicates the major alloying element or elements. The second digit represents a subgroup of the major alloy element or elements. The third and fourth digits denote the amount of carbon in the steel. For example, AISI 4140 is a molybdenum-chromium alloy steel with 0.40 % carbon content.
Carbon – Increases hardness, strength and brittleness
Chromium – Increases hardenability and corrosion and wear resistance
Iron – The principal element of steel
Manganese – Increases hardenability by lowering transformation points and causing those transformations to be sluggish and it counteracts the brittleness effect from sulfur
Molybdenum – Prevents grain growth resulting in uniformity of hardness and high strength
Nickel – Increases toughness, ductility and corrosion resistance
Phosphorous – Improves strength and machinability but has to be limited to prevent brittleness
Silicon – Improves hardness and corrosion resistance
Sulfur – Improves machinability but increases brittleness
December, 1958 – Springfield Armory begins making production M14 parts
July, 1959 – The first fifty Springfield Armory M14 rifles are assembled. The stocks and hand guards are made of black walnut.
August, 1959 – Ten of the first fifty M14 rifles are shipped to Fort Benning, GA for testing. All shipment of production M14 rifles was suspended pending test results.
September, 1959 – M14 rifle testing is completed with satisfactory performance.
October, 1959 – The first Springfield Armory production M14 rifle is presented to Master Sergeant George C. Ferguson by Secretary of the Army Wilbur C. Brucker at Aberdeen Proving Ground, MD.
January 1, 1960 to June 30, 1960 – The U. S. Army Infantry Board decides that every M14 rifle would have a hinged butt plate, plastic ventilated hand guard and bipod.
July 1, 1960 to December 31, 1960 – Aberdeen Proving Ground (MD) successfully develops a blank firing attachment and breech shield. Engineering tests on a plastic hand guard were completed at Aberdeen Proving Ground, MD.
Mid-December, 1960 – Three Harrington & Richardson M14 rifles (one receiver and two bolts) failed during range firing at Fort Benning, GA.
January 1, 1961 to June 30, 1961 – Springfield Armory makes the first production birch M14 stocks. Walnut became the alternate standard for the M14 stock.
January 11, 1961 – Ordnance Weapons Command releases Engineering Order No. 164. This document provided additional quality assurance provisions for the bolt, receiver, barrel and rifle. This Engineering Order was the result of investigations conducted by Springfield Armory, Watertown Arsenal, Rock Island Arsenal, Frankford Arsenal and Aberdeen Proving Ground into the H&R M14 rifle failures of December, 1960.
April, 1961 – The first plastic hand guards are produced at Springfield Armory.
September 21, 1961 – The M12 blank firing attachment and M3 breech shield were officially classified as Standard A.
January 1, 1962 – June 30, 1962 – The preservation procedure for birch stocks was finally adopted. It required only one dip in Class II oil whereas the walnut stocks had been dipped twice.
January 21, 1963 – Secretary of Defense Robert S. McNamara announced the end of M14 rifle procurement with that fiscal year’s contracts.
October, 1963 – Springfield Armory fabricates and demonstrates five prototype M14E2 rifles.
November, 1963 – Authority was given to Springfield Armory for 8,350 M14E2 conversions
June 30, 1964 – Official end of new M14 rifle production. TRW made its only run of M14 NM rifles in 1964.
July 1, 1964 to June 30, 1965 – Thompson-Ramo-Wooldridge, Inc. delivered the last 200 M14 rifles to the U. S. Army.
December, 1964 – Springfield Armory completes delivery of the 8,350 M14E2 rifles.
July 1, 1965 – June 30, 1966 Springfield converted 2,094 M14 rifles into M14 NM rifles.
December, 1965 – The final revision drawings for the firing pin and synthetic M14 stock were issued.
July 1, 1965 to June 30, 1966 – The following decisions were made in M14 design or production:
1) These M14E2 design improvements were incorporated into the procurement system: M2 bipod, winter trigger assembly, muzzle stabilizer with positive locking mechanism, stock back plate, better hand grip, and improved bipod jaws.
2) Design improvements were completed on a synthetic rubber stock pad to smooth out automatic fire under all environmental conditions.
3) Springfield Armory produced 2,395 M14 NM rifles by rebuilding rack grade rifles.
July 1, 1966 to December 31, 1966 – Springfield Armory completed a rebuild program on 24,000 M14 rifles.
July 1, 1966 to June 30, 1967 – Rock Island Arsenal rebuilt 2,462 rack grade rifles into M14 NM rifles.
April 30, 1968 - Springfield Armory is closed. The M14 production equipment had been crated up and shipped to Rock Island Arsenal. The M14 documents had been mailed to the U. S. Army Weapons Command.
M14 Receiver Material
All military and commercial M14 type receivers are made of AISI 8620 or equivalent low carbon molybdenum-chromium alloy steel. The M14 receiver (and bolt) should possess a hard and strong surface but ductile and tough core. The surface hardness and strength provides outstanding wear and fatigue resistance while the soft core gives excellent resistance to failure from impact. It is a hypoeutectoid steel. It is not age-hardenable. That is, it not will increase in strength below 500 degrees Fahrenheit over time. AISI 8620 alloy steel can be hardened and strengthened at the surface and made softer at the core by properly heat treating the surface. It is a free machining steel which means it forms small chips when cut. AISI 8620 alloy steel is an excellent choice for the M14 receiver and bolt.
Theory of AISI 8620 Steel Heat Treatment – The goal is to produce a part, e.g., M14 receiver, with a hard and strong surface and a ductile and tough core. This is a low carbon alloy steel. There is not sufficient carbon content to raise the surface hardness to what is desired (52 to 60 HRC) by simply heating the exterior. The part made from AISI 8620 steel can be made as hard and strong as AISI 4140 steel surface heating by carburizing. The part is placed in a carbon rich medium then heated above the A3 temperature. High carbon content (0.8 to 1.0 %) is created to a shallow depth from the surface by the diffusion of carbon into the austenite molecules present. The part is then quenched and tempered. The result is a hard, strong, and uniform depth martensite surface depth but the softer core is a mixture of martensite and free ferrite. The thickness of the martensite layer, the case depth, is much less than a steel heat treated by surface heating such as is done with medium carbon alloy steels like AISI 4140. The result is that the softer core has a larger and consistent volume than it would have by surface heating. Thus, the carburized receiver has greater toughness. This is the chief advantage of AISI 8620 steel over AISI 4140 steel for the light weight and reliable automatic M14 rifle receiver.
The following is presented as background information on AISI 8620 alloy steel. It is a low carbon nickel-chromium-molybdenum alloy steel. The composition, physical properties and description are as follows:
Carbon - 0.18 to 0.23 %
Chromium - 0.4 to 0.6 %
Manganese - 0.7 to 0.9 %
Molybdenum - 0.15 to 0.25 %
Nickel – 0.4 to 0.7 %
Phosphorous - 0.035 % maximum
Silicon - 0.15 to 0.35 %
Sulfur - 0.04 % maximum
Density – 0.283 lb/cu in
Specific Gravity – 7.8
Specific Heat – 0.1 BTU/lb/deg F
Melting Point – 2600 degrees F
Thermal Conductivity – 180.3 BTU-in/ft2-h-deg F
Mean Coefficient of Thermal Expansion – 6.6
Modulus of Elasticity Tension - 31
Typical Uses – AISI 8620 alloy steel is the most widely used carburizing alloy. It is used for gears, shafts and other applications where high wear resistance and a tough core are desirable.
Features – It is noted for a good combination of fatigue and wear resistance, hardness, strength and toughness when properly heat treated and carburized.
Shear Strength – Ultimate shear strength is about 70 % of ultimate tensile strength.
Machinability – The machinability rating is 68 % of AISI 1112 in the annealed condition. It is machined prior to carburizing so that the case depth is not reduced. It polishes well. Average surface cutting speed is 110 feet per minute.
Forming – Good in the annealed condition
Normalizing – Typically heated to 1675 degrees F for sufficient time to ensure thorough heating then allowing it to air cool
Hardening – This alloy can be hardened by 1) normalizing by heating to 1500 F then water quenching then tempering or 2) annealing then cold working.
Carburizing – Typically heated to 1650 to 1700 degrees F in a carburizing medium then oil quenched
Annealing – Anneal by heating to 1550 degrees F followed by furnace cooling at no more than 50 degrees F per hour down to 850 degrees F. Below 850 degrees F, it can be air cooled.
Forging – Forging is done from 2200 degrees to 1750 degrees F.
Tempering – The steel is heated at temperatures ranges from 400 to 1200 degrees F depending on the hardness wanted. The lower the tempering temperature the higher the hardness and tensile strength.
How was the U. S. Government Issue (USGI) M14 receiver made?
1. A slug of AISI 8620 steel is cut off from bar stock.
2. The steel slug is heated to forging temperature using automatic instrumentation. The temperature range for forging AISI 8620 steel is 1750 to 2200 degrees Fahrenheit.
3. The steel slug is placed into the impression-die forging press and formed. The raw forging is created.
4. The hot receiver forging is removed by hand and held while trimmed by machine.
5. The raw forging is then heat treated. It is normalized by heating 130 to 140 degrees Fahrenheit above the A1 temperature. This ensures the core exceeds the A1 temperature. This causes the molecular structure of the steel to change from ferrite and cementite to 100 % austenite. The raw forging is then air cooled or oil quenched and tempered at not less than 450 degrees Fahrenheit. Normalizing produces a fine pearlite structure with a minimal amount of free ferrite. The raw forging is normalized instead of annealed because it is faster and extreme softness is not needed for the receiver. Normalizing also produces greater strength and toughness than annealing.
6. The receiver goes through broaching operations. Broaching is a simple and rapid means of removing metal. Typical tolerances that are obtained by broaching are + or - 0.0005 “ to 0.0010 “. Broaching is usually more accurate and leaves a better finish than reaming or milling.
7. The receiver is machined to produce the final shape.
8. After all machining operations, the receiver is carburized, quenched and tempered. The receiver is placed in a carbon rich environment and heated to 1550 to 1600 degrees Fahrenheit. It is left in this condition long enough to obtain a case depth of 0.012 “ to 0.018 “. The carbon surrounding the receiver diffuses into the austenite structure surface. After a specified time, the receiver is immediately quenched in oil. The receiver temperature is reduced to well below the M90 temperature, 650 degrees Fahrenheit, in less than two seconds. This produces a minimum of 90 % martensite structure throughout the receiver. However, martensite lacks the toughness and ductility desired for the M14 receiver. So, the receiver is tempered at 350 to 450 degrees Fahrenheit for at least one hour. The martensite in the core decomposes gradually to a softer mixture of ferrite and cementite as temperature and time are increased. This change in the core increases the ductility and toughness of the core. The procedure is controlled to limit the free ferrite to 10 % of the core composition. By specifying and adhering to the temper temperature range and time restrictions, the amount of free ferrite is controlled. The resulting hardness and strength is achieved within the desired values.
9. The receiver is air gauged for compliance to blueprint dimensional tolerances.
10. The receiver is inspected for defects by magnetic particle inspection.
11. The receiver is phosphate coated.
12. The receiver and other M14 parts are assembled together.