When I was competing the “ Big Dogs “ mostly used Eley Black box ammo and had special chamber reamers to suit it .
My rifle seemed to prefer the Lapua / SK line of ammo which suited me because of the greater selection, with the Harrel barrel tuner dialed in accuracy was superb.......... but then the wind started to blow :)
Rimfire accuracy is an absolute mind bender with many emotional highs and lows .