i know how it works its not based on anything like the topics ive read so far it works as follows,

for the skull you need headshots
for the blue eyes you need grenade kills
for the dagger you need knife kills
for the shotgun you need shotguns kills
the strikes above your rank are rounds survived 1 strike represents 10 rounds survived in multiplayer tranzit

it all depends on how many of each you have i have a dagger 5 strikes and the skull , its not based on a genreal ranking system like more rounds the higher you are thats not true

im not entirely sure how many of each you need i hit the skull at around 500 headshots the knife is around 1000 and the shotgun is at 4000 shotgun kills thats all i no thus far