Here is the full list of all 12 GunWorld achievements.
Start the game
Unlock the Torpeagle Rifle
Unlock the B-Uzi
Unlock the Dual Magnums
Unlock the Back-A-Pack-A-Pult
Unlock the Drill Gun
Arrive at the ancient temple
Defeat the aliens and save all the guns
Surrender your guns and embrace peace, you sissy
Collect every seed expansion pack
Suffer your first Game Over
Get killed by the final boss