When you get to 500 points (Lance Corporal) you will recieve a unlock. When you get to 800 (Corporal) you will recieve another unlock after that next unlock is at 2500 then 5000 then 8000 then 20000 and then 50000
You usually get your promotion while your playing then when you quit the game the next time you load it there will be a screen telling you that you have a unlock available. You can then unlock the weapon you want. All soldier classes have 2 unlockable weapons but you must unlock the 2nd tier weapon to get to the 3rd tier. Example you have the M-24 sniper rifle the next is the M-95 Barrett after that there is the L96A1. You have to unlock the Barrett to get to the L96A1. You can save the unlock for a later date then you could have 2 unlocks at once.
http://ubar.bf2s.com/ranks.php
this site has a incredible amount of information on the ranks,badges,weapons in Battlefield 2
hope this helps
see you on the Battlefield
QcGuy (2nd LT)