Well, he was invincible. A bullet wouldn't harm him, the only reason he was weakened was because he was in the rather unfortunate circumstance of having the ultimate killing spell rebounded on him. Plus, magic plays havoc with technology, which is why a lot of pure blood wizarding families have little to no knowledge of technology, since they typically live in wizarding settlements, where so much magic permeates the area that nothing would work. And, there's many magical defensive spells. As I said, ultimate killing spell, he couldn't have defended himself from that. But a simple bullet, well there's almost certainly a permanent enchantment to stop something as small as a bullet. Plus he can apparate, so anything bigger, such as a rocket or even missile once you got desperate, he could escape in an instant.
Oh, and last time he was in power he didn't simply declare war on muggles. He never came forward, to the muggle world at least. He worked behind the scenes, similar to them destroying the bridge on his return to power, or when people assumed there was freak weather patterns such as tornados, when it was in fact the work of death eaters. Of course, they were just having fun then, it wasn't really a tactical move or anything. Since his overall goal was to rule over muggles, but he was always caught up in fighting the wizard "rebels" he might have eventually revealed himself, but only after most world leaders and important figures were under his control.