223 if you want cheap, but 6mmBR is the most inherently accurate round I have ever used.

I ran about 50 rounds through my 6mmBR never finding the accuracy, went on 6mm BR site and found an accuracy load, jammed a 105 gr scenar .015, ran loads up from a minimum just to make sure I was not overpressured, and then settled down and started shooting 3/4" groups at 300 yards with NO load accuracy work. Did not sort or weigh the brass or bullets, nothing but just regular loading with a good scale.

Setup was a normal Savage short SS action, Midway Shilein 6mm BR barrel, machined recoil lug, choate stock, SSS trigger, Farrel Steel base, Burris rings and a Nightforce 20-42x scope.

I also had a Savage 7 twist 223 before that and that rifle with Lapua brass and 90 gr bergers would shoot 1/4-3/8" at 100 yards.