tldr: the m4 and m9 both do 30 damage which is very low and requires 4 shots to kill. make the m4 2 tap chest
the fact that the m9 firing 9mm para out of a 2 inch barrel does the same amount of damage as a 5.56 out of a 14.5 is just dumb. 5.56 should 2 tap an unarmored chest imo
I dont really feel like getting into detail here but despite 5.56 being a relatively small round its speed and rotational velocity mean that upon entering flesh it instantly destabilizes and fragments leaving multiple trails through the body and a large permanent wound cavity. people very rarely survive getting shot with 5.56 in their chest cavity due to the massive internal damage it causes. (same thing applies even more so to 5.45)
also there is no issued helmet in existence that will stop any variant of 5.56