Posted by One Hole on February 13, 1999 at 17:55:58:
In Reply to: Re: .355" Wow! Thanks. posted by RG on February 13, 1999 at 16:18:54:
You're right paper does stretch, but I call it a reverse stretch. The actual hole in the paper will be smaller than the bullet diameter. But, group scorers at a br match do use outside measurement. You could argue the measuring method is measuring center to center, but it's just how you look at it. Correct me if I am wrong, but this is how I believe they measure groups. They use a device that has a circle equal to bullet diameter etched on clear plastic plate, that is attached to a caliper. This circle is placed over an edge of the group, then the device is anchored. The circle is then moved to opposite extreme edge, and this opens the caliper. A reading can then be taken from the caliper. The purpose of this device, is as RG said, to account for paper stretching. By using the circle, it's easy for a human eye to center a hole, or portion of a hole, in the cirlce. Easier than trying to determine the EXACT edge of a circle. Remember a typical group fired in competition, rarely has more than one hole. Since these devices are kind of expensive for the average shooter to use for measuring informal groups, you can do the same thing with your dial caliper. But you need a single bullet hole, not connected to other bullet holes, to do it (this is usually not a problem for most shooters). TO get an idea of how much your target is stretching, open your dial calipers to one bullet diameter. For instance, a 22 cal would be .224 (by the way, it would be .277 for a 270). Now center the blades over the bullet hole on your target. You should be able to see that the actual hole in the paper is smaller than the actual bullet diameter. The reverse stretch. Usually you will see a dark ring around the actual hole, and the edges of the calipers will usually extend to somewhere near the edges of these rings. In other words, if you just used your caliper to measure a bullet hole, you would get a wrong reading. For informal use, I just measure to the edge of the dark rings with my caliper, and subtract a bullet diameter. Guess it sounds confusing unless you've seen it done.