I can confirm XxKillface32xX's tip. Let the little kid grab the sniperrifle and the .50 sniperrifle will be unlocked...

Aaaa...
It could be. WhenI played with my friend we let the kid grab the rifle.
At home I denied him. I'll test it...

I've got a question...
I've played "Splitscreen Co-op" with a friend with a temporary profil. During that game I unlocked all weapons except the Glock 18.
Later on, at home, with my profile, the .50 cal sniperrifle won't unlock! I've played both solo and Splitscreen Co-op to chapter at the dock!

What must I accomplish to unlock it? Someachivements? Please help!