There are 5 to 10 million in america. If you do the math, 173 mass shooting deaths caused by ar15's in a decade. Lets go low and say 5 million, thats a 0.0000346 % chance an ar15 will be used to kill anyone, even lower if your talking about kids, and if you deduct the suicide rates its way less.
1700 people were murdered with knives or sharp object in the same time frame. If no one was ever murdered by a knife again, it would take 100 years for ar15's to catch up to the amount of deaths caused by knives.
327 million people in the USA, so basically you will have better odds of being struck by lighting. 490 people got struck by lighting in 10 years.