In either the simulation or arcade mode (1 or 2 players) get a high score, and then enter your name as LINUS. You will receive all weapons whenever you play after. Remember though, there is no backup battery so don't shut the game off.
To activate the CHR RAM "Smiley Test" easter egg for this game, you will need to power on the NES/Famicom console while holding Up, Down, Left, Right, B, Select, and Start. Note, however, that it is not normally possible to hold all four D-Pad buttons without specially modified controllers; standard NES/Famicom controllers will not work.
At the title screen, when the "Press Start" message appears, press Up, Down, Left, Right, then one of the following button combinations that corresponds to your desired mission. Then press Start to begin the game from that mission.
Enter these as your names in the high score table, then fill the rest with periods to get the named effect.