To become a sniper after you have compleated a few drills at the combat range and small arms range go the the sniper range. Complete 2 drills and then stand the traing brief. Pacmire or Rooker will ask if you want to become a sniper. Become a rear guard. You just have to be drilling with the shotgun and then once you get to your misson you will be assigend to rear guard in briefing. ( You better know your hand signals )
Get Sargent Rooker Angry
At the very start of the game when Rooker is briefing you with Packmire click while he is talking and he will yell at you or you could fool around during a call up and he will send you back to Metro but it won't matter.