The numbers for the 60's and 70's ran between 2.7 and 3.2 rounds per shooting....last I checked Model 10s and Police Positive Specials held SIX rounds.
Those numbers are averages. In real life, not every shooting requires emptying the firearm--in fact, the numbers tell us that many people give up when a shot is fired, even if they are not hit. So that means we end up with a lot of low numbers added in to the average.
A couple of examples.
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Let's assume 10 people shot with 6 shot handguns (no reloads) and 50% of the people give up when a single shot is fired, while the rest require between 3 and 6 shots.
1,1,1,1,1,3,5,5,6,6
Average = 3 shots. So even though 2 people emptied their guns out of the 10 in our example and one person came within 1 round of emptying the gun, the average number of shots fired was only 3 although the capacity of the guns was 6.
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Now let's run it again, but this time assume that the handguns hold 15 shots (again, no reloads). We'll have the same 2 shooters empty their guns as before but leave everything else exactly the same.
1,1,1,1,1,3,5,5,15,15
Average = 4.8 shots. Again, although 2 people emptied their guns, the average number of shots is still much less than the capacity of the firearms. The average is now higher than before even though we see that, again, most people didn't empty their firearms during a shooting.
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Anyway, the point is that increasing the number of rounds on tap will tend to push the average shots fired up given that some percentage of shootings will involve emptying the gun. Averaging in 15s where before the highest number possible was 6 will move the numbers up.
In other words, because some shootings will require more than 6 rounds to solve, and cops were given guns holding more than 6 rounds, those two facts will tend to drive the average number of shots fired up simply because in the cases where they need to shoot a lot more rounds than 6, now they CAN.