Here is the full list of all 20 Boot Hill Blaster achievements.
Score 50 points in a game.
Grab the rocket.
Grab the time freeze.
Kill 2 enemies with one shot.
Score 100 points in a game.
Score 250 points in a game.
Score 500 points in a game.
Score 1000 points in a game.
Clear level 5.
Clear level 10.
Clear level 15.
Clear level 20.
Clear level 25.
Grab health.
Grab the all-way.
Grab the horseshoe.
Kill 5 enemies with one shot.
Kill 10 enemies with one shot.
Finish a level with only 1 health remaining.
Finish a boss level without taking damage.