It is a useless attempt. First of all a max load might get you 900 fps out of a 6" barrel, but chop off 200 fps for every inch shorter than 6".
Secondly, the spread on such a shot load is about 1" per foot. So at 7 yards (21 feet) your 3 pellets will show around a 21" grouping.
Now deduct around 100 fps for every couple of feet the shot travels before reaching the target.