Unlike conventional carbines, which were generally a version of a parent rifle with a shorter barrel (like the earlier .30-40 US Krag rifle and carbine), the M1 carbine has no parts in common with the M1 Garand and fires a different cartridge. It was designed to be issued to infantry such as heavy weapons teams to provide them a backup weapon more useful in combat than a sidearm, but lighter and handier than a full-size rifle.
The M1 had a few design variants.
First production version with a full wood stock, 15 round magazine and original sights
Paratrooper model with wire side-folding stock. M1A1s were produced by the Inland division of General Motors. They were made side by side with full stock M1s and stocks were sometimes replaced by producers, making it hard to find an original M1A1 with folding stock. This variant was sometimes fielded with a vertical wooden foregrip.
A proposed variant with new sights adjustable for windage and elevation.
Named as standard against the M1A1, but may have not been issued. Came with a pantograph stock. The stock is more rigid than the A1's folding stock and folds flush.
Select-fire version capable of full-auto fire. Usually issued with an extended 30-round magazine with three catch ribs and a third magazine retaining surface. The M2 also had an altered stock, a new rear sight, a round bolt, and other small changes.
An M2 fitted with an early active infrared night vision scope, powered by a hefty backpack power supply unit. A forward pistol grip was added to assist in aiming with the heavy infrared lamp fitted to the weapon. As the T3 (T for trial, an older version of the modern "XM" designation) it saw action in 1945 on Okinawa.
The Korean War-era M3 used the M3 scope and made several improvements: the scope's detector was improved, increasing the effective range from 76 yards to 125, a conical T23 flash hider was added so that firing would not cause blinding white flashes in the scope (some late WW2 models have this too), and the IR lamp was moved from under the weapon to on top of the scope tube, as the underslung mount had proved prone to damage while the weapon was being carried.