I think it's actually "dwell time" the amount of time the bullet is in the barrel. Yes, we're talking micro-seconds when talking about the time a heavier bullet is in the barrel (due to reduced speed), but it's enough to make a difference in impact, since the barrel is ever so slightly higher by the time a heavier bullet exits. If a lighter bullet was in the barrel for the same amount of time, it would have approximately the same point of impact as the heavier bullet at the usual handgun ranges of 25 yards or less.