With the release of today's Outlaws till the End DLC pack comes 10 new achievements for you to enjoy. The achievements will add 100 Gamerscore bringing the game to a total of 1100.
What's even better is that the DLC is completely free so go get it.
Well done - 5G
Complete a Co-Op mission.
Have posse, will travel - 15G
Complete all Co-Op missions.
2 guys, 1 Coop - 10G
Complete a Co-Op mission with just 2 people.
Stake a claim - 5G
Gold medal any Co-Op mission.
Struck gold - 10G
Gold medal all Co-Ops missions.
Friends indeed - 5G
Complete a Co-Op mission without anyone dying.
You rule! - 15G
Complete all Advanced Co-Op missions.
The mother lode - 20G
Gold medal all Advanced Co-Op missions.
Dodge this - 10G
Achieve a kill chain of 10 or more in any Advanced Co-Op mission.
Bulletproof - 5G
Complete a Co-Op mission without dying.