From SMS Power!

Cheats: Line of Fire

3D mode

Hold down + while powering on the system.

Cheat Mode (Sound Test, Stage Select, Stage Skip, Extra Lives)

Using controller 2 throughout, hold down + as the title screen appears (the screen with the gun), then press any button to access and use the sound test.

This also activates a stage select (press / on the round screen), stage skip (press on controller 2 in-game) and extra lives (press on controller 2 to increase lives and on controller 2 to decrease lives).

See the main page for Line of Fire