There is an FBI or maybe ATF study floating around somewhere that was taken from "cop killers" in jail. They did the the study to see if they could find a common reason that these criminals were able to 'win' a gun fight with trained officers.
One of the things learned was the LEO's they question only spent enough time at the range to keep the qualified, time required. While the murderers reported shooting frequently, I don't remember just how frequently. But the people doing the study (either FBI or ATF) came to the conclusion that the criminals got more "trigger time" than the average LEO.
I will add they didn't seem to think it made any difference, they concluded the biggest reason the criminals won these particular gun fights was they lack of give a crap (my words) While the LEO's had to survey the situation, identify a suspect, look for a threat, observe bystanders, ect, ect. All the criminals had to do was see a LEO and fire. Then all they had to do was wound him, once wounded they could easily "finish him off." Honestly it was somewhat disturbing to read.
I found the study from a link posted by someone here on THR but I can't find it, maybe someone will have it.