According to the CDC, she is way off

In 2011 31,940 people;  In 2010 31,328 people; In  2009 31,177 people;  2007 31,224 people; and  2004 29,569 people.

That averages out to 24,800+ Multiply that by 10 and you get 248,000 for ten years.

In 2011 Accidental discharge 851 

Suicide 19,766 

Homicide 11,101 

Undetermined Intent 222 

Legal Intervention: 258 (Police shootings)

Unfortunately, the CDC doesn’t keep track of gun deaths by religious preference so we don’t know how many were caused (or inflicted on themselves) by Christians.

But it certainly wasn’t 500,000. Even if 100% of the homicides were by Christians, there is no evidence that all (or any of them) of them were ‘Gun Toting Christians’.