Type "kill" in console after getting your needed kills with the weapon, after u switch to a not
necessary weapon, type kill in the console, u getting deranked and u'll get ur previous weapon back.
E.g.: u need mp kills, finished 6 possible kills with it, now having AK. Type in kill in console - u'll back on the track with mp. You can do it 4 times before getting kicked off.
Alternatively, if possible, swap the team while a life. But also only 4x possible. Use this more than 4 times - u gonna get kicked out.