Full list of all 14 Small Arms achievements worth 235 gamerscore. It takes around 4-5 hours to unlock all of the achievements in the base game on Xbox 360.
The base game contains 12 achievements worth 200 Gamerscore, and there is 1 DLC pack containing 2 achievements worth 35 Gamerscore.
Awarded for winning 50 matches
Awarded for a score over 10,000 in the 'Shooting Range'
Awarded for winning 100 matches
Awarded for playing 100 Multiplayer matches either offline or on Xbox LIVE
Awarded for defeating 50 opponents in Challenge Mode
Awarded for defeating 15 opponents in Challenge Mode
Awarded for playing someone online who has this Achievement
Complete Mission Mode as any character
Awarded for winning 10 matches
Awarded for completing the Single Player training mode as any character
Awarded for a 5x Kill Combo
Awarded for a 2x Kill Combo
Awarded for hosting a match using the Iceberg or Village level
Awarded for completing Mission Mode as Ivar or Gullarp