There is plethora of options. The number of hours played, the number of victorious "matches" attended, killed XY enemies by gunfire, killed XY enemies by melee, etc. Any other stuff, that is usually connected with "steam achievements" (I know you dislike them, but this is a game you know
) . It could even be like: first set of achievements = NCO allowed; second set (more difficult) = officer allowed; third set (you need to invest some time into the game) = officer can revolver; fourth set = decent chance of NCO getting revolver.
Maybe with captain or major and higher backers having a shortcut.
Yes, some "veterans" do exploit the class, but significantly less of them. There is a correlation between time invested into the game and responsibility of the player. It is not 100%, but it is significant.
And you should not see it as punishment, but as a "reward". It works in other games.