John C Garand constantly tinkered with his own design and came up with a number of variations from folding stocks to box magazines.
Prior to WWII the military had conducted test, that lead to the M-1, the Garand design and the Johnson design(picked up in limited numbers by the USMC) both being full-size battle rifles, but originally spec'ed for a.276 round before much was made about the large quantity of .30 cal ammo already in inventory. As a result the 30-06 saw another work war as the standard American rifle round . The designed .276 round was not a "sub-caliber" round but was a large case round similar to the .270 sporting round. Battle rifles were still thought of as the standard.
The M-1Carbine, was a sub-sized .30 cal based on a little used Winchester round redesigned slightly. The carbine was designed as a way to more efficiently give soldiers who's MOS was anything but "Rifleman". The method at he time had been to train everyone on the rifle in basic and later, if the rifle didn't fit for their MOS they would be retrained, or worse, not retrained, and issued a pistol. It was reasoned that a light carbine, with very similar handling an operation, would be Moe efficient to issue to those already trained on the rifle..
Oddly, when the US military was making noises about having a .22, Melvin Johnson, who's rifle lost to Garand' s design, quite successfully converted the carbine to a round that had all the ballistic qualities the military wanted "for specialized,limited, use in jungle warfare" as as planned at the time. It could have been done, relatively cheaply, by converting the huge number of Carbines in stock.