There are over 100 million handguns in the United States
Assuming a future murder rate of 10 in 100,000 individuals (handguns)
Rounding down to 100 million handguns, at a 5% rate that would be 5 million murders, not including accidents and justifiable cases of homicide.
Assuming an average population of 400 million people
Approximately how long would the guns have to last before reaching a 5% murder rate?
400,000,000/100,000*10=40,000 murders per year (handguns)
5,000,000/40,000=500/4=125 years
In this case, a significant amount of today's firearms are predicted to last in excess of 100 years, and still be in use, and be employed offensively.