There are several different AR calibers on the market today. Each AR caliber has a specific purpose and can be useful for a different application from personal defense, long range shooting, hunting, to recreational shooting. Some may only require a bolt swap, while others need a complete upper swap. Here is a list of some of the different calibers you may find ARs chambered in:
5.56×45 NATO– Standard and most common AR caliber configuration. This round is going to run you about $.30 per round on average.
.223– Second most common chambering. You can use all standard AR (STANAG) magazines with this caliber. It costs about the same as the 5.56 and is very available.
7.62 NATO/.308– Fairly popular AR caliber under the generalized name of AR-10. Military uses a similar design called the M110. Good for hunting, or long range shooting. Mags for these rifles vary from brand to brand. Ammo tends to be somewhat affordable ($.50) and readily available almost anywhere.
.300 AAC Blackout– Growing in popularity for maintaining high energy at range. You can use the same bolt and magazines with this cartridge, which is one of the biggest advantages over all other alternative calibers. The ammo is slowly coming down in price and availability, but at the date I am writing this in 2016, the average price of FMJ is $.50/rd. It is pretty comparable to .308 in price but with less selection, for now.
6.8 SPC– A perceived upgrade in terminal ballistics compared to the standard 5.56 cartridge in short barreled rifles. Still can use most STANAG magazines with this caliber. It costs almost twice as much as the 5.56 at $.70/rd for FMJ.
7.62×39mm- A very affordable cartridge ($.25/rd) that can be found almost in ever corner of the world. It also sought out because it has a record of good terminal performance. Requires a whole different upper and sometimes lower. Some will work with standard AK type magazines, while others use proprietary magazines that have an exaggerated banana curve.
FN 5.7×28– Unique caliber and not very popular in the market. Cartridge is good for self defense and varmint hunting. The Ps90 magazines are used in this rifle with a hollowed out STANAG magazine used to catch or direct ejected brass. Ammo prices will vary a bit depending on your source, but will typically be around $.50/rd.
6.5 Grendel– Special caliber designed to answer a lot of issues with intermediate cartridges. Will work with STANAG magazines, but at a reduced capacity (26/30) due to the larger diameter of the brass. Can cost about as much as .308 ammo at about $.70/rd.
.458 SOCOM– A huge cartridge that is well suited for hunting big game in some cases. In a standard 30 round STANAG magazine, you can fit 10 of these cartridges single stack. Very expensive ammo to shoot ($2/rd) if you can get it.
9mm– This is a very good option for those who like pistol caliber carbines. Many times, a conversion for this caliber wont be available and you will just have to buy a whole gun chambered in it. The mags are most of the time going to be proprietary, but the ammo is very inexpensive ($.20/rd) when compared to the above selections. Also the 9mm hollowpoint can pack quite a wallop out of a 16″ barrel at close range.
There are several more special AR calibers to choose from on the market today. The great thing about the AR platform is that you can customize it in almost any way you can think of. The downside to a lot of these special calibers is that you need a completely new gun in order to run the caliber. Although, almost half of the AR caliber options I listed above were designed to answer a lot of the issues the 5.56/.223 have in terminal performance, while limiting the conversion process as much as possible. There are many roles in which we can apply our AR platforms these days. The only limitations are price, parts, and availability of ammo. The rest is all up to your imagination to decide. If you can think of it, it probably exists or can be made for your AR.