Overrated. No, actually, wrong by a two of orders of magnitude. Your link says ~30,000 deaths, NOT 300,000. also, of that number, around 55% were _suicides_ and 40% were homicides. That means around 12,000 homicides in 2000 <second paragraph>. In 2009, there were about 3 homicides per 100k people. Population was 305 million, so that's about 9,000 homicides. 300,000? Exaggerate much?
US gun ownership is about 89 per 100 people. For comparison, Switzerland has around 46 guns per 100 people and a homicide rate of 0.6 per 100k people. If you look at the list of firearm-related death statistics and compare homicide rates, and cross reference with gun ownership statistics, you'll find no recognizable pattern.
El Salvador, 50 homicides per 100k, 6 guns per 100. Jamaica 47/8. Honduras 47/6. Guatemala 39/13. Swaziland 37/6. Colombia 27/6. Brazil 19/8. Panama 13/22. Mexico 10/15. Philippines 9/5. South Africa 9/13. USA 3/89.
Do the reverse: USA 89/3; Serbia 58/4; Yemen 55/?; Switzerland 46/0.6; Cyprus 36/0.8; Saudi Arabia 35/?; Iraq 34/?; Finland 32/4; Uruguay 32/3; Sweden 32/(1.5?); Norway 31/2; France 31/0.06; Canada 31/0.8. Same story, no pattern.
TL;DR; It's not the guns, it's the crazy.