Accomplish the indicated achievement to get the corresponding number of Gamerscore points:

Red Sevens (25 points): Collect 150,000 Sheriff Stars.
Red, White & Blue Sevens (50 points): Collect 200,000 Sheriff Stars.
White Sevens (20 points): Collect 100,000 Sheriff Stars.
Three Sevens (10 points): Collect 1,000 Sheriff Stars.
Blue Sevens (15 points): Collect 10,000 Sheriff Stars.
Gun Happy (20 points): Complete a tale using only your gun.
The Cleaner (15 points): Defeat 500 enemies.
Widow Maker (25 points): Defeat 1,000 enemies.
Honorable Cowboy (5 points): Defeat a shooter without using your gun.
It's Not Easy Being Green (20 points): Complete a tale without dying.
Taking Aim (15 points): Defeat 20 enemies using manual aim.
Legend of the West (125 points): Complete hard mode and gather all unlocks & all collectibles.
Fee! Fie! Foe! Fum! (20 points): Complete Land of Giants.
The Cooler King (20 points): Complete Rodent Clan.
Forty Niner (20 points): Complete The Forbidden Mine.
Homesteader (20 points): Complete Jenkins Cousins' Homestead.
Train Surfer (20 points): Complete Water Train.
Saint Patty (20 points): Complete The Return of Rattlesnake Jake.
Hero of Dirt (30 points): Complete the game in medium mode.
Hero of the West (60 points): Complete the game in hard mode.
Why Can't We All Just Get Along? (20 points): Complete Alien Showdown.
Clatto Verata N (20 points): Complete Zombie Invasion.
Curiouser and Curiouser (20 points): Complete Nowhere & Back.
Snake Charmer (20 points): Defeat Rattlesnake Jake in The Return of Rattlesnake Jake.
The Bigger They Are (20 points): Defeat Lars in Land of Giants.
Who's Bad Now? (20 points): Defeat Bad Bill in Rodent Clan.
Thirsty Work (20 points): Defeat Bad Bill in Water Train.
Around the Bowl (20 points): Defeat Bad Bill in Forbidden Mine.
Thorough Examination (20 points): Defeat the Alien Probe in Alien Showdown.
They Got a Pill For That (5 points): Shoot Bad Bill's dynamite bundle before he throws it in Water Train.
Crate Hate (5 points): Smash open 250 crates.
100% Bull (25 points): Hit 3 bullseyes in a row.
I Shot the Sheriff (15 points): Defeat 6 zombies with one golf shot.
Bull's Eye! (10 points): Hit a bullseye in Golden Bullet mode for the first time.
Meep! Meep! (30 points): Ride a roadrunner for an entire level without dying.
Clay Pigeon (15 points): Launch an enemy with an uppercut then shoot him three times.
Good Air Day (10 points): Launch 10 enemies off the train in Water Train.
Demolitions (15 points): Lure a Jumper to land on a shotgun shell.
Sting Like a Bee (10 points): Defeat 100 enemies with melee attacks.
Tank Buster (30 points): Defeat a huge charging rabbit using only melee attacks, without taking any damage from him.
Spread the Love (15 points): Acquire the Spreadshot and take out 10 enemies with it.
McClane Worthy (10 points): Escape the exploding train in Water Train.
This is Bat Country! (30 points): Ride a bat for an entire level without dying.
Rock and Roll Cowboy (10 points): Survive the crumbling canyon in The Return of Rattlesnake Jake.
I Got a Present For Ya! (15 points): Acquire the Launcher and take out 10 enemies with it.
A Tommy Gun! (15 points): Acquire the Blaster and take out 10 enemies with it.