Caliber 102...

In today's class, we're going to be discussing rifle calibers. This is going to be a particularly difficult lesson, in that there are literally hundreds of rifle calibers out there, and trying to limit this to a "Rifle Caliber Intro" is going to be somewhat difficult. Let's start with a few generalities, toss in a few acronyms, and pick a few examples, shall we?

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Nomenclature.
Most rifle rounds are named in two parts: The number, which typically is derived from the diameter of the actual bullet, and the name, which is usually the company making the round or the person who wildcatted it first (wildcat means to take an existing caliber and change it - either by shortening the case, necking the case down further, or opening a necked case up for a larger bullet). Examples of this would be .223 Remington, .308 Winchester, ..257 Roberts, etc. Rifle rounds used in military applications are most often named using the diameter of the bullet and the length of the case in millimeters - hence a 7.62X51mm round has a diameter of 7.62mm (.308") and a case length of 51mm. Older rounds with a ##-## designation have the bullet diameter first and the weight of the blackpowder charge in grains - a .45-70 would have a 0.45" diameter bullet with 70 grains of powder behind it.

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Acronyms.
BMG: Browning Machine Gun
H&H: Holland & Holland
NATO: North Atlantic Treaty Organization (modern military caliber)
SPC: Special Purpose Cartridge
WCF: Winchester CenterFire
WSM: Winchester Short Magnum
WSSM: Winchester Super Short Magnum

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Common rifle rounds
As discussed previously, there are literally hundreds of calibers chambered for rifles. In the interest of not taking up pages upon pages by describing all of them, here are the "Top Ten" most popular rounds (limiting to 10 like with pistols). Calibers like .22LR or 9mm which are also handgun rounds will not be covered.

.223 Remington/5.56mm
.308 Win/7.62X51mm
.30-30 Winchester
.30-06 Springfield
7.62X39mm
7.62X54mmR
.375 H&H Magnum
7.92 X 57mm (a.k.a. 8mm Mauser)
.45-70 Government
.50 BMG

All of these rounds should be readily available at any gun store; all but .50 BMG should be available at most popular sporting goods chains that carry firearms and ammunition. The military cartridges can often be purchased as military surplus for much less than new ammunition, however it can often be corrosive, susceptible to misfires, or just plain old. Rifle ammunition is significantly more expensive than pistol ammunition - 7.62X39mm ammunition is the least expensive rifle cartridge out there as a rule (for new, non-corrosive ammo), and it runs between $5 and $7 for a box of 20 rounds. .50 BMG can run upwards of $4 a single round...

Rifle rounds are generally more specialized than pistol rounds - very little hunting is done with most handguns as a general rule, whereas rifles are used primarily for taking down wild game of varying size and speed. Larger calibers for dangerous game; more powerful rounds for longer-distance shooting; small, fast calibers for varmint hunting; even the diminutive .17 HMR has it's place in the hunting community for taking small fur-bearing animals with a minimum of damage to the pelt. While rifles offer far superior knock-down power when dealing with two-legged animals, they're rarely used given the danger of over-penetration. With larger, more powerful rounds available, a bullet can easily travel through multiple walls and other assorted objects. It's fine for taking down a deer out in the woods, but when you've got neighbors on three sides of you, a .300 Super Magnum with a mile-long range is going to make you mighty unpopular should you ventilate your neighbor's house with it...

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Miscellany

• Both 5.56X45mm/.223 Remington and 7.62X51mm/.308 Winchester are oddballs in that they are the same size round, but the military designations are loaded to different pressures than their commercial counterparts. Guns chambered in .308 Winchester can fire either round; guns chambered in 7.62X51mm should only fire that particular designation. It's precisely the reverse for 5.56X45mm - the .223 Remington commercial variant is lower pressure, so a rifle chambered for 5.56mm can fire both. This can lead to no end of consternation when buying ammunition for these rifles, and most often leads to folks opting for the militarily chambered arms whenever possible. Thanks to New Jovian Thunderbolt for the correction on this matter.
• Some rounds are irrevocably linked to certain firearms - the 7.62X39mm, for example, is the round used by the AK-47 and the SKS rifles. The .30-30 is a lever-action round, most commonly found in Winchester and Marlin lever guns. .30-06 is an old military round most famously chambered in the M1 Garand (but also in the Browning Automatic Rifle [BAR] made most famous by Bonnie & Clyde).
• Some rounds are "belted" - there is additional metal around the base of the case to assist in headspacing the rimless round.
  
• Short and Super Short Magnum calibers utilize a wider case that is shorter overall to maximize the powder loaded while maintaining a shorter round overall. This allows a firearm chambered in a Short Magnum caliber to have a shorter action than a traditional long-action firearm (comparing a .308 action to a .30-06, for instance).
• Rifle (and handgun, for that matter) calibers are limited to 0.50" or less diameter size under the National Firearms Act of 1934. Any firearm with a rifled barrel cannot chamber a round larger than a half-inch or it is considered a "Destructive Device" by the BATFE.
  
• Some ammunition, typically older military surplus ammo, has corrosive primers - these are primers that contain certain salts for long-term stability, and when fired, the salts get deposited on the barrel and action of the firearm being used. If the firearm is cleaned immediately with warm water and/or ammonia, the salts are neutralized and the firearm is fine. Forgetting to clean right away leads to very bad things...

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Thus concludes our (exceptionally short and barely skimming the surface) dissertation on rifle rounds. Obviously, there is a lot of material that was not covered - how the rounds are necked; why some are straight-walled and others not; why there are multiple, seemingly redundant calibers; there are many subjects simply not broached in this essay. This is intended to be a brief, skimming overview of common rifle rounds currently available, not a treatise on every long arm round ever produced.

What other information would be good for a new shooter to know about rifle ammunition?

That is all.