Start the game with the hl.exe -dev -console -game action command line.
This enables the console window. You can enter the original Half-Life missions with Action Half-Life's weapons and models. The scientists, aliens, and Barneys are the same and are fun to kill. When you enter the single player mission, display the console window and type sv_cheatsb1 to enable cheat mode. Then, type /god for God mode, /impulse 101 for all weapons, /noclip to fly around through everything, and /notarget for everything to ignore you.
Note: The game will eventually glitch.
