Here is the full list of all 26 Reign of Bullets achievements.
Reach level 100 in the Survival mode
Complete level 31 to 40 with an S-rank
Complete all 40 levels with an S-rank
Complete level 21 to 30 with an S-rank
Equip 10 epic weapons on your ship
Reach level 70 in the Survival mode
Spend a total of 1,000,000 scrap on ship improvements
Have 10 available weapon slots on your ship
Complete level 11 to 20 with an S-rank
Spend a total of 100,000, scrap on ship improvements
Clear a level on the highest difficulty
Loot a total of 100,000 scrap
Reach level 50 in the Survival mode
Wreck Titan Enforcer, the multi layered, screen filling, AI droid
Have 5 available weapon slots on your ship
Spend a total of 10,000 scrap on ship improvements
Loot a total of 10,000 scrap
Sink Drydock, the weaponized submarine, crab cyborg
Take down Daisy, the ill-tempered, trainpulling, desert worm
Use the shield to absorb a total of 50,000 damage
Complete level 1 to 10 with an S-rank
Clear a level with only 1 HP remaining
Defeat a boss with ony 1 HP remaining
Kill a boss without having weapons equipped
Shoot down Edelweiss, the flying tank dispatcher
Loot a total of 1,000 scrap