Even though it shares a very similar name, M1 Carbine (contrary to what most people think) is not merely a smaller version of the M1 Garand.
While the two rifles do indeed share the designation ‘M1’ in the name, the basic design and function of the two rifles is vastly different. That being said, these two platforms both served a critical role for the U.S. Armed Forces in World War II, and it’s important to note the main differences between them.
The M1 Garand was the standard issue service rifle for the American military in World War II. It was first adopted in 1936, as the successor to the Springfield M1903 bolt action rifle that had seen extensive service in the First World War.
While a high-quality rifle (it essentially duplicated the proven Mauser bolt action design that remains in common use to this day), by the 1930s the M1903 rifle was becoming obsolete. Bolt action rifles may have been the standard issue service rifles for all militaries at the time, but the United States military foresaw that the future lay with semi-automatic rifles rather than bolt actions.
The M1 Garand was subsequently one of the most advanced service rifles of the war. It was a semi-automatic rifle that utilized a gas operated and rotating bolt design that enabled it to reliably feed the large .30-06 Springfield rounds (marked as .30 caliber by the military, and the same ammunition that had been used for the M1903). The rifle held eight rounds and fed via an enbloc clip loaded through the top of the receiver.
The success of the Garand also resulted in numerous other militaries testing out semi-automatic rifles as well. For example, the Soviet Union attempted to replace the older Mosin Nagant bolt action rifle with the semi-automatic Tokarev SVT-38 and SVT-40 (this was put on hold following the commencement of Operation Barbarossa) and the German Wehrmacht fielded Walther and Mauser made Gewehr 43 semi-automatic rifles alongside the Mauser 98 bolt action.
Nearly five and a half million M1 Garands were built. They remained in service with the American military throughout the Korean War and into the late 1950s, until they were finally phased out and replaced with the M14 rifle (which itself was essentially an M1 rechambered for 7.62x51mm NATO with a detachable box magazine and shorter gas cylinder) and later the M16.
The M1 Carbine was developed after the M1 Garand and was a completely different weapon. While a good rifle, the Garand was also quite long and heavy.
It was therefore decided that a smaller and lighter weapon would be needed for support troops (such as radiomen or mortar men). While the shorter Thompson submachine gun was in use, it was heavier than a Garand when loaded. There was also the Colt M1911A1 pistol, but this obviously lacked range.
The M1 Carbine was developed as a new weapon that would be lighter than both the Garand and the Thompson, while offering substantially superior firepower than the .45 M1911A1. The new weapon was chambered for .30 carbine (not to be confused with the .30 caliber designation for the .30-06 Springfield of the Garand), which ballistically offered similar performance to the .357 Magnum. The standard magazine capacity was 15 rounds.
The new Carbine did borrow the rotating bolt design of the Garand, and it also made use of a shorter stroke piston that enabled better control for the shooter as well. The total weight of the M1 Carbine was just over five pounds, much less than the nearly ten pounds of the Garand.
The M1 Carbine found substantial favor with American troops, to the point that it was issued as a frontline service weapon alongside the M1 Garand, the Thompson, and the BAR. Soldiers (and paratroopers especially, who adopted a variant called the M1A1 Carbine with a folding stock) liked the M1 Carbine for its light weight, compact length, and semi-automatic firing capabilities.
Over six million M1 Carbines were built, the most of any American rifle produced during the war (even though the Garand remained the standard issue service rifle). The M1 Carbine continued to be used after the war and served alongside the M1 Garand again in the Korean War (where this time the Carbine was more commonly issued with an extended thirty round magazine).
M1 Carbines were also distributed to American allies during the Cold War conflicts; the South Vietnamese made extensive use of them in the Vietnam War, for instance.
In conclusion, the M1 Garand was the standard issue American service rifle of World War II, while the M1 Carbine was a completely different weapon that was lighter, shorter, and fired a significantly smaller round.
Aside from their similar designations, the primary factor that these two vastly different platforms shared was the fact that they each served the United States well both during World War II and in future conflicts.