The M1 Carbine has a reputation as a reliable little rifle, but in 1940 the Army rejected it twice. Winchester gambled on an impossible spec and delivered a prototype in 13 days with a cartridge nobody wanted.

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It got adopted in 1941 because the clock was ticking and nothing else weighed under 5.2 lb loaded. More than 6 million went into service by 1945.

What makes it work in the field is the same thing that breaks it today. The short-stroke tappet system uses a piston machined to 0.002 inch tolerance, but surplus parts from 10 different manufacturers push that stack to 0.008 inch or worse. Reproduction parts bake the error into the spec because no one has the original drawings. That mistake connects to magazines that tilt the follower forward and cause a failure to feed every 6 to 10 rounds. Aftermarket trigger kits advertise 3 lb pulls by stoning the sear angle, which drops ignition energy below the 26 foot-pounds needed for military primers. At negative 20°F in Korea, firing pins froze in the forward position after the first shot. Troops ran the guns almost dry and carried spare magazines inside their jackets to keep the springs warm.

Ten contractors built M1 Carbines during WWII. IBM held 0.0005 inch on critical dimensions. Quality Hardware and National Postal Meter were furniture companies that retooled in 6 months. The single part that fails most is the hammer spring, which weakens below spec after 10,000 rounds or 40 years on the shelf. Seventy-nine percent of malfunctions trace to feed lip geometry and follower tilt angle, not the spring rate. Modern steel-case ammo is harder than the extractor hook, so you get a failure to extract every 15 or 20 rounds. The carbine is a 2 MOA rifle with GI sights and a 7 lb trigger. In 2010 the CMP sold rack-grade examples for $400. The same rifles are $1,200 today because WWII collectors started buying them faster than shooters could.