As we see more people come to the site and look around the Internet in general for gun related information, and even in print, one of the hottest topics you see all over is the AR15 type rifle. There's lots of good reasons for that. One, it's one of the most distinctive and familiar guns there is, with 50 years of service in the military and dozens of manufacturers in the US offering models. Second, it's easy to shoot, so first timers can immediately feel good about it, and because they recognize it immediately when they see it in the store, it is becoming increasingly common as a first purchase. It is highly adaptable, offering lots for all different ages, sizes, skill levels, and preferences. It can change appearance and function for purpose by quick attaching and detaching of accessories. It can be built from parts to save money and directly tailor to preference by even the greenest of amateurs requiring very few tools and comparatively little time. And lastly, as we're addressing here, it can be chambered in several different calibers which can be changed in a matter of seconds by swapping out the upper half and magazine, capable of punching holes in everything from paper and tin cans to grizzly bears and everything in between.
So which caliber is for you? Or perhaps all of the above? Sure, you can just stick with one, but more is all the more fun!
1. 5.56mm/.223. This is the original chambering we all know and associate with the AR15 platform. Sometimes that's a good thing, as it's very easy to come by and useful for a lot of purposes. Lots of times in the past it wasn't a good thing, as it was thought by many to be underpowered for the military task it was given and that gave the AR15/M16 a bad rap which held it back in the US market for at least a couple of decades. These days, however, it's very well thought of by many if not most, and the offerings in different configurations makes it really adaptable.
Important note: .223 and 5.56 are the same barrel, bullet, and cartridge, but different in that 5.56 is loaded to higher pressure levels, basically a .223 Magnum or +P. Military surplus brass is a little thicker to accommodate this pressure elevation, and you can load it all the way to that or down a bit to standard .223 levels. The 5.56 chambered rifles or barrels have a little extra room and beefier construction to handle this extra pressure. You can shoot .223 in a 5.56 but would be well advised to not shoot 5.56 in a .223 marked gun.
Also note: For use of the heaviest two bullet weights, 72 and 77 grain, a barrel with a rifling twist rate of 1 in 7" is highly recommended for proper bullet stabilization. The usual is 1 in 9" which is primarily intended for 55 and 62 grain which are great general purpose and suitable for most uses, but the 77 grain really shines out past 300 yards and/or when you need to hit something with 40% more bullet than a 55 grain.
A. Varmint hunting/pest control such as woodchucks, coyotes, etc. Small feral hogs can be taken with heavier loadings like 72 and 77 grain.
B. Home defense, when loaded with hollow point and/or fragmenting ammunition.
C. General defense such as boat, RV, and property.
D. Target shooting, casual to highly competitive.
E. Dynamic carbine training, competition, etc.
2. 5.45x39 This is the Russian equivalent to 5.56/.223 for the AK74 rifle. It was developed in response to the US military's use of the M16 in Vietnam, where the communist forces were at the time using 7.62x39; the idea being that the 5.45 would be lighter, cheaper to make, less recoil and more controllable in full auto, and penetrate less therefore expend more of its energy on target rather than punching through with less wound damage. The result is a cartridge similar to the US 5.56, and has been in use for the last three decades everywhere the AK rifle has been used. Now it is available for use in the AR15 by Smith and Wesson offering M&P15 rifles and uppers.
Note: For conversion to this caliber, a .220 diameter barrel is required, NOT .224 like the standard 5.56mm takes. It may be CLOSE, but it is NOT the same and will not stabilize properly in the wrong size barrel.
Same as 5.56/.223, with the advantage of BIG savings on surplus Russian ammunition now currently available in addition to the wonderful Russian manufactured bargain brands like Wolf, Brown Bear, and Silver Bear.
3. .22 Long Rifle. The most ubiquitous and affordable rifle caliber of all. Available as a drop in kit that doesn't require a dedicated upper, so it is perhaps the cheapest alternate caliber for a rifle you already own.
A. Target and plinking
B. Small game
C. Teaching and training, particularly for new shooters
4. 7.62x39. This is the famous original cartridge of the AK47 rifle as designed by Mikhail Kalashnikov. It is identical in ballistics to the popular US .30-30 Winchester, and is equally capable as such for taking deer sized game with the appropriate soft point ammunition. The Cold War surplus market opened up the 7.62x39 in very large quantities to the US so it was a no brainer that the AR15 got its first non-.223 chambering in it. It is useful in that it is much more powerful than the standard .223/5.56, but for many years the rechambering was imperfect because the cartridge's design was difficult to feed in the AR's mechanism. Also many AR15s owned by those previously unused to using surplus ammo often fell victim to corrosion, as Soviet made ammo uses corrosive primers which if not cleaned properly immediately after shooting would rust barrels and chambers, even if chrome lined, giving the cartridge a bit of a tarnished (no pun intended) reputation in the AR system.
A. Target and plinking.
B. Hunting, up to and including deer up to 200 yards with proper soft point rounds, hollow point for varmints and predators such as coyote.
C. Home and property defense.
D. Dynamic training and competition.
5. 6.5 Grendel. Developed as a specialty round by Alexander Arms, the Grendel is intended to deliver more energy and bullet weight in the AR platform compared to the standard 5.56 (That was the one that came first and there's the most of, so yeah they pick on it a lot, ok?), particularly at longer ranges. It has a 90 to 130 grain bullet, so we're talking twice what the 5.56 is slinging. Very nice indeed. Basically it's a 7.62x39 case necked down to a .264 diameter, then formed a little different and thickened up. (Simple, yet isn't.)
Note: As this is intended for use of its extra reach, it gains appreciably from longer (as in NOT 16") barrels like 20" and longer. Remember, SPECIALTY...but very, very good specialty.
A. Target, particularly long range
B. Competition, again most useful at long range
C. Deer and antelope hunting, out to 500 yards and possibly beyond. EXCELLENT for this purpose.
D. Tactical specialty shooting (I.e. police/military sniper)
6. 6.8 SPC. Another design to deliver double or more than the 5.56 out of the same rifle, and it works really nicely. Unlike the 6.5 Grendel, it's mostly intended for 300 yards and less and does nicely out of 16" barrels, even shorter when needed. It's basically a .270 (.277, actually but think .270 Winchester as the rifle round we all know) bullet in a .30-30 case with the case fitted to work in the AR15 action. What a nifty concept! 110/115 grain bullet standard, different ones available to tailor to different needs. Shoots easy just like the "poodle shooter" but hits really hard like your usual deer blaster.
A. Target and competition.
B. Home and property defense. BIG advantage for those uses.
C. Deer and wild boar (hog, pig, swine...) hunting. EXCELLENT for such purposes within 300 yards or so, very highy regarded by hunters who use it.
D. Other uses common to .223/5.56 etc. within 300 yards.
7. .450 Bushmaster/.458 SOCOM/.50 Beowulf. Different cartridges all to do the same thing: deliver a BIG, HEAVY bullet with LOTS of energy behind it at short ranges to knock over stuff that's hard to bring down, and bring it down RIGHT THERE. Big stout cartridges, usually with a rim smaller than the cartridge body itself so that it can work with the AR15 bolt carrier, fit few to a magazine--a standard 30 round magazine for .223 fits a whopping TEN for the .50 Beowulf. Think .45-70. Up to 400 grain bullets in the Beowulf delivering over 3000 ft/lbs of energy at the muzzle. OUCH.
A. Defense, particularly against dangerous targets.
B. Hunting heavy game like bear, elk, or moose at close range.
C. Lots of fun showing off how big your......barrel...is at the range. Blast stuff like pumpkins, 55 gallon drums, bowling balls, etc. It's FUN. LOTS OF FUN. Expensive fun, but FUN.
So there ya have it, folks. There are a couple more here and there like the .204 Ruger and 5.7x28 (pointless in any state with magazine capacity restrictions, but very fun for the 45 normal states), oh and all the little pistol caliber cartridges too like 9mm (EXTREMELY POPULAR), .45 ACP, and even 7.62x25 Tokarev (SINFULLY CHEAP TO SHOOT AND OBSCENELY FUN!!!), and I can get into them, but that's A Brief Guide Volume 2...if it were in Volume 1, it wouldn't be brief, as in A Brief Guide, now would it?