Master Weapon

I am at something 5, World 10. Does the master weapon shoot out the little green balls as ammunition? And how do I change the weapon that appears at the bottom right side of the screen? I have pressed x and z but nothing happens, though the weapon on the bottom left side does change. I am really tired of getting killed, but I would like to achieve World 11.

The master weapon uses plasma as its ammo. Yes, the same plasma you were using as money up til now, and you get it the same way you ever did - kill things or collect plasma bonus powerups. If you've run out of plasma by firing when the bosses' shields were up, then you're in trouble - the only way to get more now is by killing those tiny fighters it spits out every cycle.