Adding my $.02.
Replaced factory barrels (recent manufacture, two dimples) in two guns with Apex 5" semi-drop in. Results:
M&P9Pro C.O.R.E.: before 4.94", after 3.21".
M&P9Pro: before 4.87", after 3.36".
Those are averages of 8 groups 10 shots each, excluding worst shot in each group, at 25...