all of the above are correct; center to center of the farthest spread.
usually I get three to four tight and then they start to open up from a heated barrel,
that's because I get impatient waiting for the barrel to cool down.
I usually shoot a full magazine, whatever that is (3,4,or 5 shots), same with pistols
or revolvers(full cylinder or magazine)
The size of the group is all that counts, there are no scoring rings on the target. Using a micrometer, the group size is measured at it`s two farthest points (largest outside diameter). The diameter of the bullet (in thousandths of an inch) is subtracted from that measurement to produce the true group size. Shooters fire from sturdy benches with the rifle supported by a rest and may have as long as 15 minutes to fire their group.�