Well, pick a brand. Military FMJ ball is not usually impressive for accuracy. If you buy Winchester or Remington you'll never know in advance what country is was made in, and you can, as another thread pointed out, actually get different diameters from one lot to the next because the contract switched suppliers. I've not shot Winchester .223 FMJ, but their 147 grain .308 FMJ's are just awful. A gun that holds .5 moa with match bullets will not hold 3 moa with the Winchesters in my testing, and it didn't matter what I did to tweak the load. The only thing as bad were some pulled Lake City M2 bullets I had from a lot in which you could see the bullet bases had three distinct forming die tooling marks. Good FMJ's that I've shot are Sierra and Hornady.
Midsouthshooterssupply.com was a good source for the Hornady in bulk, but I haven't looked to see what they have in stock these days? I've also bought Hornady .308 FMJs in bulk from
Midwayusa.com, but only when they had a sale going on. Otherwise their shipping will kill you.