The U.S. Caliber .30 Carbines

The U.S. Carbine Caliber .30

The Parts






Trigger Housing

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Understanding the Part Variations & Methods used to Identify Them

Knowledge of the part variations and when they occurred is used by many owners and collectors to evaluate a carbine's authenticity. Knowledge of the historical circumstances that created these variations and how these changes were implemented by the various prime contractors and their subcontractors provides a perspective often lost when viewing these differences as black and white.

The variations and their value in assessing a carbine have warranted a system by which they can be identified and universally recognized.

The methodology used by U.S. Army Ordnance to assign part numbers to the various parts and their variations during and after WWII has not been amenable and/or logical in differentiating one part variation from another. Their methods are described in more detail at the bottom of this page.

Since WWII a number of authors have attempted to simplify identifying the different variations by coming up with their own number and/or letter systems. This has produced several different methods that have lacked the simplicity required for remembering them.

The standard chosen by collectors and owners over the years was developed by author Craig Riesch for use in his book U.S. M1 Carbines, Wartime Production and it's subsequent revisions. Given the existing universal acceptance of this standard it has been chosen as the standard used on this website.

Riesch's system separates the part variations by assigning a number to each variation in the general chronological order it appeared (i.e. type I, type II, etc). Under this system some of the parts chosen as Type I were eventually found to be prototypes or of very limited production as part of the research and development that preceded mass production and in some cases the Ordnance technical drawings. We have chosen to explain these particular variations instead of rewriting the standard.

The rest of this page is devoted to the general historical circumstances that created these differences and the realities of mass production at 10 different facilities during a time of war.

Early Part Variations: Research & Development prior to Mass Production

The Winchester prototype adopted by the U.S. Army Ordnance Department on 30 Sep 1941 as the new "Carbine, Caliber .30, M1" was a design concept built using parts and experience gained from manufacturing a variety of other firearms. The fast track development of the Winchester prototype left a number of issues that needed to be addressed before the carbine started full production, including the engineering drawings providing the standards and details necessary for mass production.

On 05 Oct 1941 Winchester and the Inland Division of General Motors were tasked separately with building five pilot models for submission to U.S. Ordnance for testing in addition to interim engineering drawings. During this time Inland and Winchester produced a number pilot models, tool room models, and/or prototypes with one or more parts specific to a particular carbine or a limited number of carbines.

Winchester Prototype
Serial Number 0

Winchester Prototype Trigger

Winchester Prototype Sear

By the end of January 1942 U.S. Ordnance had tested the ten pilot models making alterations where they were needed and had produced a master set of engineering drawings.


One of the standards set by Ordnance was interchangeability from one carbine to the next regardless of who manufactured the part. Another was, regardless of whatever changes were made to a part, it had to be backwards compatible with all parts previously produced.

Every part design or redesign went through extensive testing before the design or change was adopted. A minimum lifespan for each part was established and measured by the number of rounds fired before the part failed. To ensure parts met these standards, samples of each part and entire carbines were fired by Ordnance personnel under a variety of conditions until the samples failed. Throughout production, random samples were occasionally pulled from productions lines so the parts could be interchanged with those from other manufactures and test fired. That many of these parts are still in use over half a century later suggests the minimum standards were more than adequate and/or exceeded by many manufacturers.

Prime Contractors, Contractors, & Subcontractors

Between November 1941 and January 1943 ten separate companies were contracted by U.S. Army Ordnance as prime contractors to produce .30 caliber carbines. None of the prime contractors had the means to manufacture all of their own parts. Each subcontracted dozens of smaller companies for the production of various parts. Throughout production the demand for parts led Ordnance to contract dozens of additional companies for the production of parts for use as spares and replacement parts. A number of these companies were subcontractors already producing a part or parts for one or more prime contractors.

The facilities of all of these companies along with their existing machinery, tooling, and production capabilities were evaluated first hand by Ordnance personnel. Prior to the beginning of the war the machinery, tooling and personnel of most of these companies served many different industries unrelated to firearms production. The age and technical capabilities of machinery varied from one company to the next. The needs of an entire nation moving into massive wartime production caused shortages that prompted many companies to become creative in the redesign of existing machinery or machinery that could be obtained.

While all finished products had to meet Ordnance specifications the differences in machinery and tooling sometimes created slight variations specific to a particular manufacturer. One example was the trigger housings manufactured by Irwin-Pedersen. With the inheritance of the Irwin-Pedersen contract, Saginaw Steering Gear acquired the Grand Rapids facility and used the same machinery and tooling, continuing to produce trigger housings with the same characteristics.

White arrow indicates tool mark common to the trigger housings manufactured by Irwin-Pedersen
followed by Saginaw Steering Gear at the Grand Rapids facility. Black arrow indicates tool mark
sometimes present. Blue arrow indicates repair peening for tighter fit of housing to receiver,
not specific to those made by Irwin-Pedersen and/or Saginaw in Grand Rapids.

Ordnance specifications initially set the height of the two vertical grooves in the rear wall
of the magazine well at a length sufficient to accommodate the two small nubs on the rear of
the magazine as they sat atop the magazine catch. Ordnance approved the full length grooves
as optional to allow for the various types of machines being used to cut the grooves.

Ongoing Research, Development and Implementing Changes

Efforts to cut production costs, improve or simplify manufacture, address whatever issues arose and experiment with new ideas were ongoing throughout production and well beyond the end of WWII. These efforts produced a number of changes to various parts throughout WWII and afterwards.

When design changes were accepted by U.S. Army Ordnance they were forwarded along with the Ordnance drawings to the prime contractors and subcontractors with instructions related to implementation. In theory, all of the manufacturers would then make the changes forthwith.

In practice, the prime contractors and subcontractors evaluated what would be needed to implement the changes. The changes could involve acquiring new materials and/or additional machinery, changes in mass production lines, retraining personnel and a variety of additional challenges that required varying levels of resources and time. During a time when resources were often in high demand and short supply.

Given the circumstances of the time, introducing change to mass production lines operating at peak production at ten different prime contractors and their subcontractors spread across thousands of miles of America sometimes inevitably resulted in varying degrees of implementation. Some of the manufacturers may have been able to make the change(s). Others implemented the changes in phases. Some could not make the changes or could implement only part of the changes. This produced variations specific to certain manufacturers. Ordnance approval in November 1943 of a number of changes that included the introduction of the round bolt was an excellent example. Additional details are available on the page devoted to the Bolt Group.

National Postal Meter bolt production area

During WWII newly designed parts were not replacement parts. When the new parts eventually made their way to the assembly line, earlier variations were still used until existing inventories were depleted. When an assembly point required replenishment of a particular part, whatever version was available was literally dumped into the containers they withdrew parts from for assembly. Often the parts of the older design in the bottom of the container were used after parts having the new design. If a quantity of parts with an earlier design were located at a later point in time they were simply added to the container(s) for assembly.

Ordnance personnel conducting inspections and repairs both during and after WWII received Modification Work Orders (MWO's) directing that a certain part or assembly be altered or upgraded. The alteration and/or upgrade was dependent on the necessary tooling and/or availability of the parts.

Understanding Ordnance Part Numbers

Standard Nomenclature System (SNL)

(SNL) 7 Digit

Federal Stock Number (FSN)

National Stock Number (NSN)
Ordnance manuals, publications and packaging printed between 1941 and 1974 underwent a number of changes in relation to part numbers that can be confusing. These changes were the result of revisions to the means by which part numbers and stock numbers were assigned during this time period. Understanding these changes helps eliminate most of this confusion.

The Standard Nomenclature System (SNL) 1920's-1954

The inventory system in use by the U.S. Army Ordnance Department when WWII started was called the Standard Nomenclature System (SNL). Under this system engineering drawings were assigned a letter from A-E to denote the drawing size. When a part design was adopted it's engineering drawing was assigned a number. Numbers were issued numerically and unrelated to the item the part was for. The drawing number became the part number. If a redesigned part was adopted it's drawing was assigned a new number that became the part number for the newly designed part.

As an example, when the carbine flat bolt was adopted the drawing/part number was C57148.

Under the SNL system, packaging containing parts were marked with the part number preceded by a code identifying the item the part was for. The designation for the M1, M1A1, and M2 carbines was B028, sometimes shortened to B28. Thus packaging containing one or more flat bolts was marked B028-C57148 or B28-C57148.

At the beginning of WWII the U.S. Army Ordnance Department maintained the inventory for every part and every item they were responsible for. Trucks, howitzers, small arms, tanks, aircraft and more. Every part down to a nut, bolt or screw was assigned it's own SNL number. Ordnance maintained a separate inventory system for ammunition.

Midstream Metamorphosis

The massive influx of newly designed and/or redesigned ordnance items during WWII revealed a number of weaknesses with the SNL system. To an extent that warranted change during WWII with the goal of eventually replacing the SNL system.

In mid 1943 Ordnance changed their numbering system. Newly adopted designs were issued a seven digit number, numerically and unrelated to the item the part was for. All existing part numbers in the SNL system were changed to a seven digit number by replacing the letter at the beginning of the part number with a value added to the remaining numerical number. A added 5 million, B added 6 million, C added 5.5 million, D added 6.5 million, and E added 6.9 million.

The flat bolt previously identified as part number C57148 became 555-7148 (5,500,000 + 57,148). Every part number in existence in the SNL system, including all parts for the U.S. Carbine, Caliber .30, was changed. One of a number of problems with this conversion process was it failed to address the conversion of part numbers having a letter at the end of the sequence. This resulted in seemingly random part numbers absent the logic provided by the use of a known standard.

Some of the challenges implementing changes introduced by U.S. Ordnance during wartime production were discussed in the previous section. The result in this case was an increase in the use of the seven digit number system by the end of WWII but the changes didn't become universally standard until the implementation of the Federal Stock Number System in 1954.

Federal Stock Numbers (FSN) 1955-1974

The various different inventory systems previously used within the Department of Defense were finally consolidated into one with the final implementation of the Federal Stock Number system in 1955.

Federal Stock Numbers consisted of 11 digits. The last seven digits remained the same as introduced in mid 1943 and indicated the part number. The four digits preceding the part number grouped the parts within a general category related to the items they were used for.

The flat bolt that started as C57148 with packages marked B028-C57148 or B28-C57148 had become 5557148 with packages marked B028-555-7148 or B28-555-7148 by the end of WWII and later. Under the FSN system the flat bolt became 1005-555-7148. The number 1005 denoted weapons from 1mm through 30mm. Package markings were now 1005-555-7148.

National Stock Number (NSN) (1974-current)

By the mid 1950's NATO had established a National Codification System (NCS) for inventory control. This system simply added a two digit country code between the first four digits and the seven digit part number of the Federal Stock Number (FSN).

The National Stock Number (NSN) implemented in September 1974 brought the United States and NATO systems together using the first four digits of the FSN followed by the country code followed by the part number.

The country code used by the USA is "00" or "01". "00" generally indicates an item in US inventory before 1974. "01" usually indicates an item placed in US inventory after 1974. The "00" country code has been in use by the United States as a NATO member since the mid 1950's, concurrent to the use of U.S. Federal Stock Number absent the country code until implementation of the NSN system in 1974.

Most NATO country codes used prior to 1974 have been changed.

The U.S. Carbine Caliber .30 flat bolt assigned 1005-5557148 under the FSN system became 1005-00-5557148 under the NSN system. The part number in use by NATO since the mid 1950's.


This information is offered to assist in understanding the confusing part number designations that appear in various Ordnance publications and on packaging printed since 1941. Parts having their part designation on the actual part were not changed to the later stock number.

Should you have questions, assistance is available on our Discussion Forum.

The Discussion Forum also serves as a reference desk for the more advanced material that could easily overwhelm a website and is often subject to opinions that may vary
due to a lack of original documentation. A number of researchers and authors are present on the forums, helping others and seeking information for various research projects.