That's a national average....agencies that spend more time/money on training do better.
The best performing police department I have seen data wise is Baltimore with a hit percentage around 50 percent.
50 percent of officer involved shoots occur at 5 yards or less, 70 percent at 8 yards or less, and 75 percent at 10 yards or less. Beyond 7 yards accuracy plummets.
But as far as rounds fired, it's complicated.
There was an increase of rounds fired per officer per engagement from around 3.5 to 4 over the period where the switch from revolvers to semi auto pistols occurred.
Officers on average shoot more when more officers are present. The average shots fired per officer per engagement increase with the number of officers present. However while multiple officers are present at 75 percent of of officer involved shoots, multiple officers only fire in about 33 percent of officer involved shoots.
However in a subset of those multiple officer involved shoots there's a group shoot effect where officers start shooting and keep shooting because other officers are shooting. That leads to ridiculous numbers of rounds being fired at a single suspect who went down after the first few rounds.
Despite those outliers, the data indicates 75 percent of officer involved shoots are concluded with 6 shots or less. That rises to 80% with 7 rounds or less. Beyond that it's unclear whether or not the time required to reload a revolver proficiently with a speed loader would make a difference. 91% are resolved in less than 12 rounds. Only 3.8 percent require 15 rounds or more.
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Despite all the efforts beginning in the Reagan era who hype threats against officers, a factor in the adoption of semi auto pistols in the US, there's a reasonable argument that could be made that with improved training (including better open hand skills, better de-escalation skills, etc), a revolver would still be sufficient for police officer use, particularly a 7 shot revolver in .357 like a 686+.