So I'm taking my rifle (5.56/.223) out to the range here soon. It's a range that goes all the way up to 500 yards, so I want to try shooting at various distances upon getting the new optic zeroed (4x fixed magnification) at 100 meters/yards (I don't know how the distance markers are set at this range). The barrel is 1/7 twist, 17" barrel. I want a couple different quality brands and varieties of ammo to test for accuracy. They don't necessarily have to be match grade, but whatever is recommended. If I can get it online, then that's the best option.

Bonus question: I need a good online source for a good deal on cheap plinking ammo, for non-accuracy shooting practice and familiarization.

I'm open to any and all suggestions based on your own experiences and reviews.

I spent an afternoon testing a bunch of 223 and 5.56 commercially available and easy to find ammo.  In my admittedly casual testing protocol, I discovered that Australian outback 223 in 69 gr shot superbly through my 16" 1:7 twist barrels.  In a 55 gr bullet I found Federal American Eagle Military Grade shot really well. I did not try the Australian 55 gr version.

There might be some 69 gr and 77 gr that could shoot better, or a reload would likely be better still, but I was within .5 MOA with AO 69 gr.  And these are regular old AR uppers and barrels.  

YMMV of course.

What was the 69 gr stuff called? Do you remember the brand and variety? I might pick up some bulk American Eagle M193 also, if I can find it. What about American Eagle M855 bulk packs?

Each barrel has a preference, but I’ve always had the best luck with Black Hills 77 SMK’s. Whatever the answer is, it’ll usually be some loading or another of the 69/77 SMK projectile. Hornady 75’s have always been a wild card for me, sometimes they keep right up with the SMKs and sometimes they’re barely ahead of ball. To contrast that, I’ve had almost universally positive experiences with the SMK.

At over a dollar per round, they better be, lol. I'll see if I can find a box of 20, to start off with. My goal here is to test different varieties to see the best grouping.

My 1/7 loves the Hornady Black 75bthp. Not cheap but easily 1moa ammo and on the good days even submoa (~0.7moa)

As for 55gr plinking ammo Fiocchi, Sellier Bellot, GGG, AE are all good.
The GGG and AE are quite accurate too at under 2moa