Army of Two - 40th Day - Unlock the the .50 sniperrifle

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!

Mine unlocked when i let that little chinese kid get that sniper rifle for me hope this helps

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...

Mine unlocked when i let that little chinese kid get that sniper rifle for me hope this helps

XxKillface32xX

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