Well, you can usually find some winchester, it is good, you can find it at Wal-Mart for a pretty good price. There is also Black Hills and Hornady, but these are usually too expensive for my taste. I do like American Eagle, it is federal ammo that is pretty cheap and it is a great round. I have also heard that they have started using Lake City Brass for it, so that is even better. You can also try Wolf, but I wouldn't recommend it, espically if you want to collect the brass and sell it or reuse it, because Wolf has steel cases and is not reloadable. I would rather shoot brass, collect it, and the put it on E-Bay and get some more money to buy more bullets and then continue the cycle. Or if you have a friend that reloads, they may give you some if you bring them the brass.
If you gun has a 5.56 barrell, then I would defantly say go for some Q3131A or some XM193(Lake City) Ammo. Both are really great rounds and they are really cheap.
But the best advice I can give is to try out different ammo and see what you gun likes and then buy it.
I hope that this helps. Also the link that has already been suggested is great.
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