What is a carbine?

Answer 1

What it really comes down is manufacturer's designation, but a carbine is typically shorter than a standard service rifle of a country which is manufacturing the carbine. For example, in the First World War, Germany used the long Gewehr 98 rifle. In the 1930s, a shorter version was developed and produced - the Karabiner 98 carbine. The M1 carbine was much shorter than the standard M1 Garand rifle, but didn't meet the criteria of a submachinegun, so it was designated as a carbine. Another interesting example... when the SKS was first fielded, it was designated as a carbine, as it was considerably shorter than the standard Soviet weapon of the time - the Mosin-Nagant 91/30. However, when the AK-47 - which was shorter than the SKS - went into production, it was classed as a rifle, since it was to become the new standard armament of Soviet troops. Likewise, the Chinese followed suit - the Type 56 carbine was a Chinese-made SKS, and is designated as a carbine, whereas the Type 56 rifle is shorter than the SKS, and is classed as a rifle.

Sometimes a carbine will use smaller cartridges than the standard issue rifle; sometimes it will not. The M4 carbine fires the same cartridge as the M16 rifle it supplements, while the SKS carbine fired the smaller 7.62x39 cartridge as opposed to the full sized 7.62x54R cartridge which was standard fare for the Soviet armed forces when the SKS was first fielded.