I will grant you that Geometry was a long time ago, and my memory might not be up to snuff, but
as I remember a 6" coaster would mean a 3" radius, and the formula, C = 2πr would mean that the distance around the circle is 18.85 inches
12 shots around the circumference is not going to make it with a 50 cal.
Then there is the issue that if you had pinpoint aim and the center of the 50 cal bullet hit the line, the remaining part would be smaller than a 6" circle, so the radius for the aim point would have to be 0.25" larger, and thus a larger circumference requiring yet more bullets.