Dennis Henigan tells us guns aren't useful as a deterrence against violent crime:
Apart from the statistics, the deterrence theory poses an interesting conundrum. If criminals are deterred by the prospect that their victim may be armed, how can we account for attacks by armed criminals against other armed criminals? Why do armed drug dealers have anything to fear from other armed drug dealers? Why do armed gangs have anything to fear from other armed gangs? Pro-gun researcher Gary Kleck of Florida State University reports that street gang members are over eight times more likely to own handguns than other youths, and nineteen times more likely to be homicide victims. Drug dealers are almost four times more likely to own a handgun and six times more likely to be homicide victims. Why doesn’t their gun possession deter attacks on these criminals? Surely it can’t be true that bad guys fear only armed good guys, but not other armed bad guys.
Half-Truth Henigan, as is usual, only explores a subset of the situations. The missing part is where one side is armed and the other is disarmed. How much deterrence is there then?
It's obvious Henigan is only a lawyer for the Brady Campaign instead of an engineer, a scientist, or a carpenter. If carpenters built house like Henigan builds his theories of criminal behavior the first woodpecker to come along would destroy civilization*.
* I'm plagiarizing
Wienberg's Second Law.