The wonderful world of .223 Remington/5.56x45 mm NATO ammunition. First off for the budding reloaders .223 is actually a .224 diameter round. I was surprised when I discovered that years and years ago since I figured caliber numbers were equivalent; therefore, .223 must = .223, nicht whar? Nein, .223 Remington = .224 diameter bullet.
Questions to ask yourself: What barrel twist does your rifle have? What chambering is your rifle? And lastly gas system over egged or in spec?
1/15, 1/14, 1/13, and 1/12 twist rates on a .223 Remington will fire the sub 55 grain bullets. These are usually the varmint hunting bolt-action .223 rifles that shoot rodents out to 800 yards utilizing a 45 grain bullet.
The ever common 1/9 twist rate. This is the most common twist rate for rifling in .223 Remington firearms, and as such odds are your AR-15 (unless you are a snob like me) has this rate of twist. This is good for stabilizing common and inexpensive .223 Remington ammunition with weights of 55 to 69 grains. It also is good at throwing cheap M885 surplus NATO ammunition down range.
1/8 and 1/7 twist rates are not as common but preferred. 1/7 is the government standard for M16 and M4 rifles.It has the ability to stabilize the 70 to 80 grain rounds. Typically this is where the boutique ammunition resides in the market such as Mk 268 Mod 0, 5.56x45 mm NATO 77 grain SMK, Barnes TSX 70 grain, or the well known Hornady 75 grain Match and Tap ammunition line.
AR-15 rifles come in three different chamberings. The two most common are those in .223 Remington and 5.56x45 mm NATO. The third least common is .223 Wylde.If you are chambered for the last then fire whatever you want be it .223 Remington or 5.56x45 mm NATO. If you are chambered only in .223 Remington then stick only with .223 Remington, and if the rifle is not properly chambered to what it claims or you want it to be chambered in 5.56x45 mm NATO instead, find a gunsmith and have that person ream your chamber. (See the last two pictures: Bushmaster AR-15 and my frankenbuild INFIDEL. I traded off the Bushmaster due to the rear sight being off, but that gun was one of the few Bushmasters marked 5.56 and meant it, but it has a 1/9 twist barrel; so, it cannot stabilize the higher grain bullets, which I prefer. INFIDEL on the other hand has a LaRue Tactical 16" DMR upper, which has a 1/8 and a proper chambering. She shoots .5 of MOA at 100 yds with that 4 MOA Aimpoint Comp ML2 sight: shooting 62 grain BHTP Prvi Partizan to 77 grain SMK ammo loads.) And so you know .223 Wylde is what competition shooters utilize its purpose is to provide a chamber tightness good for both .223 and 5.56.
As for the gas system: some companies over egg the hole for the gas system. This is so you can shoot off all that cheap Russian garbage without having it fail to feed. The drawback to this is that when you put in (most) normal Western ammunition you begin beating on your BCG due to too much gas pushing back on it.
As for what AR-15 to purchase? I like Colt, LaRue, Noveske, BCM, Daniel Defense, LMT, and KAC. My experience mostly is with LaRue Tactical, Colt, LMT, and Bushmaster, and thus my opinion.
As for ammo uses: there is practice/training/plinking loads, hunting loads, competition loads, and defense/duty loads. Use what is appropriate to task at hand.
So you know the AR-15 is my second favorite rifle, and yes I dropped money on ammo big time. My first is the pre-64 action line of bolt-action rifles, such as my pre-64 Winchester Model 70.
(Edited to add: fotos from my Hornady reloading book.)