Heres a solution i did it, it may take while getting headshots but its better than waiting for an invite to a unlock all challenge lobby when you hardly get invited, here is some step to take.

1. Get the bot infection by joining someones session
2. Back out and go to system link and start game
3. Choose class and end game straight away
4. Go to private match and change map to whatever you want DONT change the match type, then change game settings to headshots only etc.
5. Then start the game and you should get the bots in private match, it has worked first time everytime for me but if it doesnt just dashboard and start over (just becareful when you end the game and want to do other guns you will need to dashboard and then start from step 1 again).

This worked for me last night as someone told me how to and i so far have Gold AK,Uzi,M1014,LMG just need the Dragunov now

Please Thank and +Rep