Less than 1% of guns are ever used to cause harm. There are several factors that have to be looked in making this calculation.
1) There were 279,417 gun crimes reported to police in 2019 (https://ucr.fbi.gov/crime-in-the-u.s/2019/crime-in-the-u.s.-2019/tables/table-19 and https://ucr.fbi.gov/crime-in-the-u.s/2019/crime-in-the-u.s.-2019/tables/expanded-homicide-data-table-8.xls). The number increases to about 400,000 when you use the National Crime Victimization Survey numbers.
3) How many guns are there? The Small Arms Survey, which is relied on by the media and many academics, puts the number of guns in the US as 400 million in 2017. Data on household ownership of guns is available here.
4) How many times is each gun used in a crime? The best guess is at least three times on average, but that is clearly a lower bound.
In any given year, that means that there are about 0.035% of people harmed by guns. Over a 25 year period you are talking about something around 0.4% to 0.88% of guns are involved in such incidents.