Based on Tom Givens' presentation regarding the 60+ defensive gunfights his students (Memphis TN area) have been involved in, the AVERAGE gunfight was 3 shots at 3 feet or less and was over in 3 seconds.
But, I thought he put that in proper context by saying not to put too much stock in averages. For one thing, if you prepare for the average that means you're under-prepared half the time. For another, you can have one foot in a bucket of ice and the other in a campfire and on average you're comfortable. But if you don't want to get burned, you'd better be able to handle a fire even if on average you don't need to.
Of these DGUs that Tom Givens' students encountered, at least one of them involved stopping a man 25 yards away who was crouched behind a car. Another one of the DGUs was 11 shots and the reason it wasn't more was because the gun went dry at 11 rounds and there wasn't time for a reload. The average gun fight was 3 rounds/feet/seconds but that is just an average.
All that said, nothing wrong with a 638 and I carry one too. There are times when that small of a gun is all I can readily carry given how I have to dress sometimes. When I'm able to dress more to my liking though, the 638 becomes the BUG and a more conventional autoloader gets carried too. Yet another line I remember from Tom Givens' presentation was the observation that, in debriefing these private citizen DGUs and a number of police gunfights, he said that (paraphrasing) no one in a gun fight ever wished they had a gun that shot a less powerful round or held fewer rounds of ammunition.