I am pretty sure that Benning school for boys allows civilian weapons in military courses so not sure a flatter cartridge is going to give you an advantage. You are going to end up with the issue 308.
However, caliber wise I would suggest a 7 or 30 cal for reasons stated. I would suggest you look at the 300 WSM for this reason. It is seeming to have 2x or more barrel life of any other magnums. We have guns with 4000 rounds and still competitive in BR. That record is phenominal compared to other mags.
Savage 12VSS in 300 WSM is around $550, 26" heavy barrel. Go with that to start, Ferrel 20 MOA base, rings ($150 total) and good scope at $500 or less.
Join the Quantico Shooting club right below you and start shooting their modified Fclass matches and come to the VA 1k BR matches at Quantico this spring.