Insector X.
 No.  Code  Effect
   1  AVZA-AA3G  Keep shot power after dying
   2  AVZA-AA3N  Keep special weapons after dying
   3  BVSA-AA2W  Infinite stock
   4  AVWA-AA2W  Infinite credits
   5  AAFA-AAFY  1 credits
   6  AEFA-AAFY  2 credit
   7  AJFA-AAFY  3 credits
   8  AYFA-AAFY  6 credits
   9  BJFA-AAFY  11 credits
 10  CTFA-AAFY  21 credits
 11  BJDA-AAD4 + BJBT-AABA  Start with 10 stock
 12  B6DA-AAD4 + B6BT-AABA  Start with 15 stock
 13  CTDA-AAD4 + CTBT-AABA  Start with 20 stock
 14  DEDA-AAD4 + DEBT-AABA  Start with 25 stock
 15  GJDA-AAD4 + GJBT-AABA  Start with 50 stock
 16  NTDA-AAD4 + NTBT-AABA    Start with 100 stock
 17  9JDA-AAD4 + 9JBT-AABA  Start with 250 stock
 18  8TDA-ACD4 + 8TBT-AABA  Start with 500 stock
 19  7ADA-AGD4 + 7ABT-AABA  Start with 1,000 stock
 20  BKZA-AABY  Starting speed of 10
 21  B7ZA-AABY  Starting speed of 15
 22  CVZA-AABY  Starting speed of 20
 23  D3ZA-AABY  Starting speed of 30
 24  EPZA-AABY  Starting speed of 35
 25  GKZA-AABY  Starting speed of 50
Don't combine Codes 26 thru 29--when shot power goes above 6, the game will freeze.
 26  SFYT-BNT0  Each P worth 3 (half) shot power
 27  SFYT-B2T0  Each P worth 6 (full) shot power
 28  SFYT-BJT0  Each P worth 2 & each PP worth 3 shot power  
 29  SFYA-B218  Each PP worth 6 (full) shot power
Tips: The game plays to your level.
If you have a lot of shot power, the enemies will be harder to kill.
In some cases, it's better to have LESS shot power, because the enemies will go easier on you!
Also, the speed codes have the greatest effect on difficulty, not the shot power codes.