Alright so I'm progressing threw the challenges of using assualt rifles only averaging 2-6 head shots a game and I realize what you need to do in order to unlock the camo
1. 100*9 head shots ='s 900 head shots
2. 10*9 longshots ='s 90 long shots
3. 150*9 no attachments ='s 1350 kills with no attachments
4. 150*9 no perks ='s 1350 kills with no perks
5. 10*9 Blood thirstys "5 kill streaks" ='s 90
6. 20*9 double kill medals ='s 180 double kill medals
This is a game where you play at least 8 months but I do already have 4 guns gold and my lowest guns headshots average around 30 but it does seem time consuming and a difficult task to achieve. It is really aggravating using the guns that don't matter like the Swat and the Fal or "Fail" and I just hate to have to go threw the grind to get them all. Who ever has accomplish this should be proud because it is a very time consuming challenge. I think there are 63 challenges that have to be completed to get diamond camo for your assualt rifles.
PS: I'm kinda drunk correct me on my math or ignore it but I did the math on an Iphone calculator.
Good luck and HAPPY KILLING :-)