LOTS OF POTIENTAL HERE!
You have all the elements of making a good game here but it's overshadowed by lack of lifespan: Everyone could go on for ages playing this and still think its just as good as the first time you shot a BLAM down. Its too easy and needs to be more challenging.
Just go back and either alter this to more options, levels, special stuff (double shots, instant aim, power pack charger, etc...) and of course an boss (big blam with twin lasers firing back, Tom Fulp's head, etc...)
Just with them would make this game in the top 10. No seriously.