I use a 200 gr .40 cal xtp in my 45 ML and a 300 gr .44 cal in my 50. They are plenty accurate. My 40/45 combo is capable of 1" to 1 1/2" groups at 100 yds. And that is with open sights out of a T/CRenegade with a GM barrel.
As for shooting 200 or 300 yds. It is probably accurate enough but you are really asking a ML to do a lot after 150 - 200 yds. At 200 and especially 300 yds and anywhere in between the holdover would be critical. And I am not sure how much energy the bullet would have when it got there.