ORIGINAL: BigTiny
If you can roll your own, then 550 would be a good buy.
If you are looking for brass, find out where your local LEO's go to shoot. They drop plenty of new brass shooting that qualifying course with the AR. Most dept.'s favor public rifle ranges like a wildlife management area. Call the rangemaster on site, sometimes police shoot on days the range is closed to the public. Bring a couple of buckets.
The LEO (PD. county and HP) all use the same range I shoot at.
I pick up brass everytime I go there whether I shoot the caliper or not.
I hate to see the good stuff get stepped on or go to crap in the weather.
I have several reloading friends that we trade brass because we don't all shoot the same calibers.