Firepower 2000

Return to the title screen:
Press Start, then hold Select.

Control both vehicles in one player mode:
Highlight the two player option on the title screen, then press Start + A.

Level skip:
Pause game play and enter one of the following codes while playing in the corresponding level.

From level 1 to level 2
Press R, L, R(2), L(2), R, L, R(2), L(2).
From level 2 to level 3
Press R, L, R(2), L(2), R(3), L, R(2).
From level 3 to level 4
Press L, R, L(2), R(2), L(4), R(3), L.
From level 4 to level 5
Press R(3), L(3), R(2), L(2), R, L, R, L.
From level 5 to level 6
Press R, L, R, L(3), R(2), L(2), R, L, R, L(2).

Hint: Level 1 bonus:
Move to the far right at the start of level 1 to locate a yellow container. Blast it open to collect bullet tokens.