It's difficult to make accurate assumptions about 500 foot-tall fictional monsters whose very existence violates the laws of physics. But it's liable to have skin, scales or other outer with protective blubber or equivalent covering several feet thick. This will absorb anything except apart from an armor-piercing round. Flesh, like water, can stop virtually any projectile within a few feet – that's why you need something very exotic like a supercavitating round if you want to go through a lot of it. Those supercavitating Russian APS underwater assault rifles might be handy here... but you'd need a lot of rounds to have any effect.
Link to part one, link to part two. -via Metafilter