The system I will describe uses the top of the animal’s back as the standard aiming reference point, with the goal of delivering bullets to the vertical center of an animal’s body. In order to accomplish this task, we need three additional pieces of information: the number of inches the front bead covers at various ranges; the amount of bullet drop that is present at each range; and the animal’s backbone-to-belly measurement. Let’s address each of these factors separately, before putting all of this together.
The front bead I recommended may measure 3/32″ in diameter when you hold it in your hand. However, once it’s installed on the barrel, it will project (or subtend) a significantly larger circle, depending on how far away from your eye it is located (i.e., the length of the barrel) and the distance to the target. We need to know these “bead covers” measurements at proposed shooting distances, in twenty-five-yard increments. Since target coverage by the bead usually isn’t severe enough to interfere with aiming inside of 100 yards, let’s start this exercise at 125 yards. A square of plain white paper can be used as a backdrop. As a beginning point, make the square ten inches per side. With the gun in a stable position, place the target exactly 125 yards away, using a laser rangefinder to determine the distance. Sight down the barrel at the white square and note whether the front bead just fits within the square, is significantly smaller than the square, or is larger than the square. Adjust the size of the white square until the bead just touches the sides when looking down the barrel, and note this “bead covers” measurement at 125 yards. You have it right when the bead’s relationship to the target looks like the middle picture of Figure 1. The same procedure should be repeated at additional increments of range. Of course, as the distance to target grows, the bead will cover an ever larger circle. Although your “bead covers” measurements may differ, for my muzzleloader the results are: 10 inches @ 125 yards, 13 inches @ 150 yards, 15 inches @ 175 yards, 18 inches @ 200 yards, 20 inches @ 225 yards, and 23 inches @ 250 yards.
The next piece of information to be gathered is the amount of bullet drop that can be expected at each of these distances. As you shall shortly see, in order to provide a useful aiming convention at extended ranges, we need to measure this drop in relation to the top of our front bead. Before advancing on this front, however, we first need to zero the gun. We’ll do that at 100 yards. The front bead can be expected to cover about eight inches (give or take) at this distance, which is substantially smaller than the backbone-to-belly dimension of most big-game animals. At 100 yards from a rock-solid position, zero the gun so that the resultant group is centered left to right, and falls slightly high of center, vertically. For this exercise, aiming can be facilitated by selecting an appropriate target. A square piece of white paper, which is an inch or two larger than the projection of the front bead will work nicely. Thus, if the bead covers eight inches at 100 yards, a white square measuring nine or ten inches per side will make it easier to center the front bead while aiming.
Once the rifle is so zeroed, we’re going to determine the amount of bullet drop at each of those ranges where we previously obtained “bead covers” measurements. Once again, plain white paper will suffice as a target. While shooting this time, however, it’s important to hold the top of the bead at the top of the paper. Doing so will provide drop values in terms of how far below the top of the bead (BTB) shots fall at each range. As examples, my BTB measurements are: 7 inches BTB @ 125 yards, 9 inches BTB @ 150 yards, 14 inches BTB @ 175 yards, 24 inches BTB @ 200 yards, 33 inches BTB @ 225 yards, and 41 inches BTB @ 250 yards.