The system has been, too be honest, fairly buggy, with multiple patches needed to sort it out so any of us could actually unlock weapons (and further patches to stop people unlocking them by idling/spectating servers using third-party hacks).
After a lot of flak, Valve seem to have relented and reintroduced the achievement grind milestones for the Sniper & Spy alongside the random unlocks – whilst this has unfortunately meant that the servers have re-devolved into Sniper/Spy fests, people playing classes that prey on such weak targets can still unlock further weapons by simply playing normally!
Carry on…!
