Assuming you prefer brass over steel I would recommend lake city 5.56 Nato, American Eagle (black box) 5.56 or Remington .223. Cabelas, Midway, Sportsmans Guide, Slick Guns etc usually have 100-500 round bulk cases for between 32 - 50 cents per round for 55gr. 62gr green tips are slightly higher, either will work for your planned use.
The Tula, Bear and other ruskie steel cased ammo is a little cheaper, trade-off is a dirtier round and a bit more clean up to keep things working flawlessly. Also, some ranges do not allow the use of the steel casings, no salvage value for them since you can't reload it and they have to sort it out of their brass scrap buckets.
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"A free people ought not only to be armed and disciplined, but they should have sufficient arms and ammunition to maintain a status of independence from any who might attempt to abuse them, which would include their own government." - George Washington