Here is the full list of all 21 Guns, Gore & Cannoli achievements. It takes around 6-8 hours to unlock all of the achievements on Windows.
Complete the game on impossible mode
Survive The construction site without killing any enemies with weapons.
Survive the sewer using only your Lupara
Survive the river level using only your magnum
Survive the army stronghold without killing any enemies.
Kill 20 enemies in the alleystreets, during one run, by kicking them into fire.
Complete 'Bonnino's Stripclub' without dying once on Normal Mode
Consecutively kill 25 enemies with a headshot
Complete the game on normal mode
Complete the game on hard mode
Complete the game on easy mode
Defeat Mickey
Kill 2000 enemies
10 multi-enemy grenade kills
Defeat Sonny
Kill 1000 enemies
Destroy 10 cars
Kill 400 enemies
5 multi-enemy grenade kills
Eat 10 Cannoli
50 headshots