there are doubtless some lives saved throughout the year because someone had a gun; we don't know how many.
But what is undeniable to any rational person, is that that number is dwarfed by killings by accident, on impulse, or suicides. There are probably more "burglars" shot and killed by accident who were in fact innocent - like the recent incident where a man shot and killed his son because he was dressed strangely - than there are genuine criminals deterred from crime.
There are 100,000 shootings a year in the US, 30,000 killed. Of these many are suicides, many of them people who acted on impulse who may well not have done so had a gun not been readily available. There are huge numbers of wives or husbands killed with a firearm who would be alive today if they hadn't been shot on impulse.
There is also one child shot by accident every two or three days.
It probably is too late to put the genie back in the bottle; the NRA are simply too powerful. They even effectively ban the gathering of statistics to study gun ownership effectively.
However, if we look at the example of Australia, following the Port Arthur massacre, in which the killer used an assault rifle, there is hope.
A right-wing administration focused on the most dangerous weapons; roughly one fifth of all the weapons in circulation were returned, and payment was made for them. it is also possible to limit the amount of ammunition, to limit the number of weapons kept at home (we hear the killer's mom had four or six). THere are also sensible measures like limiting the trade at gun fairs, where at the moment it's easy for criminals or the insane to buy guns. The Australian legislation cut down gun crime to a massive extent, and there have been no mass killings since then.