Honestly, what runs well in one weapon may not run well in another...
Many people recommend
http://www.hornady.com/store/5.56-NATO-75-gr-BTHP-Superformance-Match/ for 5.56
or
http://www.hornady.com/store/223-rem-75-gr-bthp-match/ for .223
I tend to be a quantity guy, rather than quality when it comes to 5.56 so am not the best about high quality ammo.
I'd try many different ones and see what works well.
Ohhh so many variables to consider. Every gun and ammo combination is different. First you need to define "what kind" of rifle (e.g. AR, bolt rifle, etc.) are you interested in? What are you really going to do with it? Upright, mobile action shooting (google 3-gun), hunting? Hunting b/c you're going to carry it up a mountain? Or just from the car to a blind? Hunt what? Are you going to take practical/tactical training classes and rig it out all tacticool with lights and lasers? Or just shoot it from a benchrest at a very stationary piece of paper, hoping to make tiny holes right next to one another?
Once you figure out what you're going to do with it, you then choose the best style, weight, barrel profile, etc. to suit your needs. You won't want a 24" bull barrel for 3-gun or tactical entry. You don't want a 14.5" lightweight pencil barrel for long range sessions. Optics are a subject unto their own. Open/peep sights, red/green dot optics (magnified or no), and all sorts of scopes ranging from low-power non-adjustable, to variable length and high-power scopes that often cost more than the gun they're on. Again - let your actual needs drive your research and buying. Don't just get any old gun and then figure out it's the wrong thing for what you want to do.
Once you get the gun that suits your needs, you move on to the next step. "Dialing it in."
It'll be up to you to figure out what ammo a) actually works best in your gun, b) what you can actually afford to feed it and accept the results, and c) just run & gun "plinking" ammo. Any/all must run in the gun (cycle the action if semi-auto), feed and fire reliably. Any ammo that doesn't goes off the list.
On a 5.56 or .223 caliber rifle, they come in different twist rates, ranging from 1:7 (in theory bullet spins one full rotation every 7"), 1:8, 1:9, and 1:12. You don't see 1:12 much any more, as it won't stabilize the more common, heavier bullets. You'll want a faster twist rate (1:7) if you intend to fire mostly match grade 68 or 77 grain bullets to make small groups on paper. If you're just going to shoot 55gr or 62gr fmj at metal plates, you could go with the more-common 1:9. My AR is chambered in .223 Wylde and has a 1:8 twist rate. It shoots almost everything in both 5.56 and .223 very accurately. I tried well over a dozen factory loads. All group less than 2". 55gr vmax is <1" 53gr Superformance Varmint is < .75" if I do my job.
I added the Ruger mini-14 last month, had a scope kicking around so I installed it, and have since added a 6" Accu-strut on the tapered barrel to deal with heat distortion. I can constantly hit 8" steel at 100 yards. I'm really enjoying shooting this rifle.
It's the law......
