Here is the full list of all 28 Steam and Metal achievements.
Destroy 24 aircraft continuously
Accomplish level with rank F.
Beat level 1.
Level beaten without air-torpedos.
Accomplish level without lossing a single life.
Destroy 100 aircraft in one level.
Beat level 2.
Beat level 3.
Beat level 4.
Beat level 5.
Complete the game.
Destroy 69 aircraft continuously
Destroy 111 aircraft continuously
666 aircraft shotten.
1337 aircraft shotten.
10000 aircraft shotten.
Achieve accuracy 25%
Achieve shooting accuracy 50%
Achieve shooting accuracy 77%
Level is beaten without a single scratch.
Kill 30+ enemies with a single(!) air-torpedo.
Beat the game on hard.
Pass through level without kill anybody but boss.
Take 1000+ bonuses on level.
Accomplish level with rank A.
Destroy 200 aircraft in one level.
Destroy 300 aircraft in one level.
Beat game with score one million or more!