Boy is your guestion a hard one to answer easily. You need to define a class to shoot in and build to the clas rules. A quality barrel on a well tuned action rangeing from bolt to semi-auto. Shooting class again. For a cartidge at your minimum yardage a .223 can be a winner on up. Not so much about what cartidge as it is finding the right mix of components but the 6mm and 6.5 do seem to win many shoots today. Here is a good site and some idea of whats needed for atleast one world class rifle and loads, after that its the shooters issue. http://www.6mmbr.com/gunweek051.html-
And it s sister site- http://www.accurateshooter.com/
Now guys do use basicaly tuned stock rifles and can compete in hunter class's at local matchs and egg matchs , enjoy and learn too. Savage builds some good rifles with most of whats needed to come up a step with a rifle out of the box with there target class rifles that might be a good way to start and learn with. In a year or two you can go to a aftermaret barrel.