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Rocky’s Reloading Room |
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Matched Loads
Have you ever wished you had two loads for a given rifle? A heavy hunting load for big game and a lighter load for varmints or practice? That’s a pretty common wish and one that’s easy to accomplish. But let’s add one other factor that seems to throw in a clinker: the two loads should have the same trajectory.
The lighter load will allow us to practice with lower recoil and blast, but with the identical trajectory, so we will instinctively learn the correct hold at all ranges. It may even shoot to the exact same impact point, so we can use the same sight-in settings. With no changes except ammo, we’d have “an all-seasons, all-reasons” rifle.
Now that starts the chin rubbing and the head scratching, doesn’t it? It may seem difficult, but there’s actually a simple way to have just that.
First, let’s review what causes a bullet to fly in a particular trajectory, or arc, from the muzzle to the target. Without getting into all kinds of esoteric factors like gyroscopic precession and stuff, there are only two things that control trajectory: velocity and drag. In simplest terms, the faster an object starts out, the farther it goes before it hits the ground. And, the sleeker it is, the longer it keeps its initial velocity. Obviously, the two things interact.
If we throw an object at a leisurely speed, it doesn’t matter how sleek it is, it can’t stay in the air for long. If you lob a rifle bullet underhanded, it goes across the room and that’s it. Launch it from a gun at three times the speed of sound and it goes for five miles.
Drag is the other factor we mentioned. Fire that same bullet where there is no air, and it would go on darn near forever. But fire it in air and it slows down quick. The controlling thing here is the amount of drag or air resistance the object has. As we learned in high school physics class, weight is not an issue. Drop two objects of the same shape but different weights, and they will hit the ground at the same time. But drop two objects of different shapes, and the air will retard one more than the other. The sleeker one will hit the ground first. An easy example is a parachute. Drop an unopened parachute and it will plummet to the earth at a speed of perhaps a hundred miles an hour. Open that parachute and it will descend at less than ten miles an hour. The weight is the same, but the shape of the ‘chute changes the drag of the air immensely.
So it is with bullets, although not as dramatic. A long, pointy bullet with a tapered base has much less drag than a short, blunt bullet with a square base. If we start those two bullets at the same speed, the sleeker one will fly farther than the other. We call that a “flatter” trajectory.
We define the amount of drag for a given bullet shape with a number called the ballistic coefficient. That’s simply a numerical rating for how sleek a bullet is. The higher the number, the less drag a given bullet has. A theoretically perfect bullet has a rating of 1.000. One only half that good would have a rating of .500, and so forth. Common bullets have ratings from as low as .150 to just a tad over .500, a pretty good spread.
That’s enough theory. Let’s start loading!
The way to have two loads with the same trajectory is to select two bullets with the same ballistic coefficient and then fire them both at the same speed. Because those are the only two controlling factors, that’s all we have to match. With careful selection of the bullets and load data, it’s actually pretty simple.
If our big game bullet is a standard spitzer softpoint with a flat base, it’ll probably have a ballistic coefficient (BC) of about .400 or so. The first step is to find a varmint-weight (lighter) bullet with about the same BC. The new plastic-tipped ones available today fall right in that range. It’s not necessary to match the BC down to three digits, because BC tends to change a bit anyway, and the numbers are not exact. If one bullet is rated at .430 and the other at .420 or .440, that’s close enough.
Now we will match the velocity. The big game load is almost certainly loaded to near top end for best range and performance on game. Varmint loads are usually loaded to near maximum also, but that’s not what we want here. We want to duplicate the heavier bullet’s muzzle velocity. If the big game load has a muzzle speed of 3,000 fps, that’s what we want for the other bullet as well.
Usually, that’s a reduced load for the lighter bullet, but that’s easy to do. Just check the tables of load data for that bullet and find a load that matches the desired speed. Look at the “Start” loads for several powders and you should find one that is within a hundred feet per second of the big game load. It almost certainly will use a different powder than the full-power load, but that’s hardly a handicap.
Close is good enough for a starter. Later, if we want to, we can tweak the velocity of the lighter load a bit to compensate for a BC that’s not an exact match to the big game bullet, but that may be a bit too compulsive. Out to a few hundred yards it really won’t matter all that much.
We may tweak the seating depth of the light load a bit for best grouping. That’s a standard loading procedure, though.
All that’s needed is to shoot the two loads at paper to see if they hit at the same point of impact. Fortunately, that is often the case. If the velocity is about the same, barrel harmonics tend to be about the same as well. That leads to both loads having the same sight-in setting – or nearly so. If the setting is off by a couple of clicks, it’s an easy enough thing to remember. But if the two loads hit within an inch of each other, it’s probably best to leave the sight-in setting alone. You won’t have to remember which load you were sighted for.
And there you have it. You now have a matched pair of loads that shoot the same. But one has dramatically less recoil and noise. You can practice, plink or varmint hunt with the light load all year long. The hold over for any range quickly becomes a matter of instinct. You hit more targets with the first shot, you become more confident, and you enjoy shooting even more.
There is a saying about shooting: “Beware the man with one gun; he likely knows how to use it.” We might now amend that old truism a bit. Beware the man with one gun - and two loads!
Rocky Raab ©Copyright 2002 |