Between 1 and All
If anyone knows, they’re not telling.
I keep seeing 500,000 a year but can't seem to kind a source or citation anywhere for it which makes it rather suspect. (If you don't know a fact manufacture it) I found the value of firearm thefts on FBI.gov but not the number of thefts.
This
USA Today article (2002) shows "The number of guns stolen per year has declined, from 221,322 reported in 1993 to 138,035 in 2001"
It also says 'nearly 1.7 million firearms were reported to police as stolen from January 1993 through August 2002.' This averages out to 196,153 (and change) per year.
Those numbers in the article come from the Americans for Gun Safety Foundation which is from the 'only safe gun is a melted one' camp but still falls well short of the 500K total that pops up uncited everywhere.
'Average number of firearm thefts that occur every year in the US: 341,000
(Source: US Dept. of Justice, Bureau of Justice Statistics, Guns and Crime, 4/94)’ (Can’t find it either, likely not digital) from a PBS Frontline transcript.
That's all Google, Yahoo! & Bing could come up with in an hour.
Found this thread already.
{Found this gem too at PBS, 'Percentage of L.A. High School students who say they could obtain a gun for less than $50: 25 (Source: ACLU report: From Words to Weapons, 3/97)'}