this is what i think. a gunblade is a lot like the vibroblade from star wars. when you pull the trigger of the gunblade, bullets filled with kinetic energy emit the energy into the blade, causing it to vibrate extremely fast thus making cutting things easier. for those who wish to argue about the bullets and reloading this i how it works. when ever squall swings his gunblade the kinetic energy he creates fills any empty shells in the cylinder.

mind you this is only an idea of how it works.