First whip your pistols out then hold melee down LT, after you melee twice keep hold of LT and press Y+RT.

You should now be holding LT+RT+Y and your gun should have changed, hold the buttons for about 8 seconds then release LT but keep hold of RT. You should unload a whole mag in one shot.

This only works with the Uzi & Assault rifle. Works wonders against Tanks and you can even one hit kill a witch with the Uzi!