Simple common sense suggets that if a .308 diameter bullet cuts through the paper, the hole HAS to be .308 in diameter. I have just taken an unfired .308 AMAX bullet and slowly lowered it though one of the holes in my last target….it completely fills the hole even out to the edge of the grease ring. So, why would I want to subtract .290 instead of .308?? Maybe, I just don’t understand…….

EDITOR: Next time you’re at the range shoot one bullet, all by itself, into a clean target paper. Assuming you get a nice round hole, measure the hole outside edge to outside edge. Almost certainly that measurement witl be SMALLER than the nominal bullet diameter. This is because spitzer bullets (unlike pistol wadcutters) do not create a clean hole like a punch exactly one caliber in diameter. Instead part of the paper is removed, part is deflected, and part flexes and rebounds. Since you actual hole size is less than the diameter of the bullet, you need to use that for future calculations.

Going back to that one bullet hole, you can see why. If, for example, the measured single hole on paper is .290 (not atypical), and you shot a second bullet in EXACTLY the same hole, and then you subtracted .308, you would end up with a negative number for the group size, and that’s impossible. Conversely if you subtract the actual group size (with two bullets in the same hole) you’ll get a .000 or “zero” group, which is the best you can do. If you are just subtracting one bullet diameter from edge to edge measurement, you’re probably under-stating your group size by .10 to .15 inches. I recently measured a target for a Forum member. He marked it as a 0.302″. In fact it measured 0.463″ using a center-to-center plot with a scanner.

Randy has a new website. Well, it’s actually a year old now, but it’s got more info than the old one.

http://www.bibullets.com/

I know this because I set it up