.223 Rem. Cheap brass. Cheap bullets and efficient. If tight twisted, can work well on medium game.
I put over 1000 rounds thru my 223 RAR in the past few months, mostly out to 500+ yards. Been using the 75gr AMAX at a measly 2900 fps. Not a ton of rounds, but I think I've learned more with that cheap 223 than all my other rifles combined. Amazing for such a low round count.
Shooting in wind with minimal recoil has been a great learning tool. I've seen a bud make some great shots with the same rifle, after struggling with more recoil from more "manly cartridges" at half the distance. Trigger time and round count make up for speed and BC to provide a path to shooting skill.
I've been late to the 223 & 75 AMAX party so might be preaching to the choir
