Side Arms Special is, in essence, the same game as the regular edition of the arcade side-scrolling shooter Side Arms (with audio CD tracks instead of MIDI music) - plus a new gameplay mode called Before Christ.
This mode can be considered a "hardcore mode", since the player has to return to the beginning of the level after the player character has died, as opposed to losing a "life" and continuing from the same spot in the regular mode. Also, while in the regular mode the player can change the type and direction of weapon (laser, spreading, etc.) at any time, in Before Christ mode, it is possible only upon picking up a corresponding power-up.
Another change includes the ability to charge the weapon, similarly to the feature in R-Type. The charged beam is more powerful, but obviously requires more time to execute.