How do I get thermal optics for weapons in battlefield 3?

I was playing bf3, I killed a guy who was assault class, picked up his kit, he had an F2000 with thermal optics! I want it, but how do you get thermal optics for a weapon.How do I get thermal optics for weapons in battlefield 3?
IRNV? You get a whole lot of kills for whatever weapon.How do I get thermal optics for weapons in battlefield 3?
Just keep killing people with your weapon, you'll get it eventually. Every ten kills you get something for your gun (except when you get 100 kills, it goes by twenty five then possibly fifty).How do I get thermal optics for weapons in battlefield 3?
just keep on killing people and getting points with it and you unlock attachments for that gun. thermal is a boss whth silencer and fore grip
you have to get points and use the gun that you want to get thermal optics for

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