Tedly,
Until other instances come forward, the stats are fairly simple, even if just an estimate.
Here are some numbers:
There were over 14 million (14,033,824 ) gun purchases last year (2009)www.fbi.gov
From November 30, 1998 - February 28, 2010, only 761,814 have been denied. www.fbi.gov
Here are the stats for Total NICS Background Checks for the same period: www.fbi.gov 112,380,272
0.06% Denials.
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With that as background, now let's consider this particular case. The number of guns that are sold by police departments that end up and are used by criminals. For simplicity's sake, we won't worry about guns bought this year and used next year or after.
We know of exactly 2 cases where guns sold by the Memphis Police Department were used in a criminal act.
We do not know how those two guns came into the hands of the shooters. Were they stolen? Purchased at the gun shop? (If so, the individuals, if they were not allowed to own guns, would have been denied by the NICS background check, since we know that is so effective. (/sarc) )
So now we need to know how many firearms were sold by various law enforcement departments across the U.S.
We know in Kentucky, the State Police sell them at auction. And we know that they had about 500 firearms (after you remove the pulled, BB guns, etc.) for one auction.
They also have six auctions scheduled this year (www.kentuckystatepolice.org )
So for just Kentucky, since I haven't found data yet for Tennessee, we've approximately 5000 firearms up for sale this year alone. I estimate it lower that 6 x 500, just to be conservative.
That means using JUST Kentucky's data, we'd have 2/5000 = 0.04%. { or 2/6000 ( 0.03%) for a liberal interpretation. }
And since we know that more than one state's law enforcement departments sell firearms, the percentage goes even lower.
