While I spend more time teaching handguns, students sometimes have questions about rifles, so this will be a very short tutorial on that subject.
Rifles have a much longer history than handguns, as man has been obsessed since at least the 14th C. with finding new ways to kill the enemy. Development has gone in many directions, always embracing new technology, from the muzzle-loading smooth bore flintlock and the “rifled” musket, to the machine gun. The European wars of the late 19th C. ushered in the age of the bolt action rifle, which was the dominant combat weapon until WWII (although still common for hunting and sport). The semi-automatic has been dominant since about 1950. The full automatic (machine gun) actually predates those, going back to WWI, but initially they were not very portable, and not very reliable. Why does this matter? Because we now understand that the big bullets still fired from bolt-action rifles will tear up the guns that cycle rapidly. Semi-autos now frequently use cartridges with small bullets, but a lot of gunpowder.
There are now hundreds of rifle calibers. Here are some very common ones, with notes:
5.56/.223 AR-15 rifle. The two calibers are the same, with different powder load.
.30-30 WIN One version of a 30 caliber cartridge. The 2nd number is length. WIN = Winchester
6mm CM A popular hunting cartridge. CM = Creedmore.
.243 WIN Another hunting cartridge, slightly larger than 6mm.
.45-70 GOV Very old, almost obsolete. GOV = Government.
7.62×51 (AKA .308). Hunting of larger animals. Used by U.S. for combat (Afghanistan).
6.5 CM Slightly larger than #3 above.
7.62x54R Russian combat cartridge (1897-c. 1950). Similar to the German Mauser. WWII.
.270 WIN Smaller than the last four above, but much faster. Better ballistics.
.30-06 (30 aught 6). A legendary, and powerful, rifle cartridge. WWII.
.338 Lapua Legendary long range sniper cartridge.
.50 BMG Very long range sniper cartridge.
There are a few applications where rifles are built to shoot pistol ammo, usually 9mm. They are called Pistol Caliber Carbines (PCC). The famous Tommy Gun of the Prohibition Era fired .45 ACP. The Heckler & Koch MP5 submachine gun, used by Special Forces around the world, fires 9mm.