A total of over 6.1 million M1 carbines of various models was manufactured, making it the most produced small arm for the American military during World War II (compared with about 5.4 million M1 rifles and about 1.3 million Thompson submachine guns).
Despite being designed by Winchester, the great majority of these were made by other companies (see § Military contractors below).
The largest producer was the Inland division of General Motors, but many others were made by contractors as diverse as IBM, the Underwood Typewriter Company, and Rock-Ola Manufacturing Corporation.
Few contractors made all the parts for carbines bearing their names: some makers bought parts from other major contractors or sub-contracted minor parts to companies like Marlin Firearms or Auto-Ordnance.
Parts by all makers were required to be interchangeable.
Often one company would get ahead or behind in production and parts would be shipped from one company to the other to help them catch up on their quota.
 When receivers were shipped for this purpose the manufacturers would often mark them for both companies.
 Some of the strangest combinations were the M1's made by the combined efforts of Underwood and Quality Hardware, resulting in the manufacturer mark UN-QUALITY.
The receiver was subcontracted from Union Switch and Signal, not Underwood.
Many carbines were refurbished at several arsenals after the war, with many parts interchanged from original maker carbines.
True untouched war production carbines, therefore, are the most desirable for collectors.

The M1 carbine was also one of the most cost effective weapons used by the United States military during World War II.
At the beginning of World War II the average production cost for an M1 carbine was approximately $45, about half the cost of an M1 rifle at approximately $85 and about a fifth of the cost of a Thompson submachine gun at approximately $225.
The .30 Caliber carbine ammunition was also far cheaper to produce than the standard .30-06 ammunition; used fewer resources, was smaller, lighter, faster and easier to make.
These were major factors in the United States military decision to adopt the M1 carbine, especially when considering the vast numbers of weapons and ammunition manufactured and transported by the United States during World War II.
A total of over 6.1 million M1 carbines of various models was manufactured, making it the most produced small arm for the American military during World War II (compared with about 5.4 million M1 rifles and about 1.3 million Thompson submachine guns). Despite being designed by Winchester, the great majority of these were made by other companies (see § Military contractors below). The largest producer was the Inland division of General Motors, but many others were made by contractors as diverse as IBM, the Underwood Typewriter Company, and Rock-Ola Manufacturing Corporation. Few contractors made all the parts for carbines bearing their names: some makers bought parts from other major contractors or sub-contracted minor parts to companies like Marlin Firearms or Auto-Ordnance. Parts by all makers were required to be interchangeable. Often one company would get ahead or behind in production and parts would be shipped from one company to the other to help them catch up on their quota.  When receivers were shipped for this purpose the manufacturers would often mark them for both companies.  Some of the strangest combinations were the M1's made by the combined efforts of Underwood and Quality Hardware, resulting in the manufacturer mark UN-QUALITY. The receiver was subcontracted from Union Switch and Signal, not Underwood. Many carbines were refurbished at several arsenals after the war, with many parts interchanged from original maker carbines. True untouched war production carbines, therefore, are the most desirable for collectors.
The M1 carbine was also one of the most cost effective weapons used by the United States military during World War II. At the beginning of World War II the average production cost for an M1 carbine was approximately $45, about half the cost of an M1 rifle at approximately $85 and about a fifth of the cost of a Thompson submachine gun at approximately $225. The .30 Caliber carbine ammunition was also far cheaper to produce than the standard .30-06 ammunition; used fewer resources, was smaller, lighter, faster and easier to make. These were major factors in the United States military decision to adopt the M1 carbine, especially when considering the vast numbers of weapons and ammunition manufactured and transported by the United States during World War II.
M1 carbine