ONCE THE BULLET leaves the muzzle of the rifle it is subjected to the force of gravity. No matter how high its muzzle velocity, every bullet follows a downward curving path. This causes a problem for the hunter when his bullet kicks up dust under an animal’s belly instead of hitting where it was aimed. All bullets have almost exactly the same rate of fall. If, for example, you hold a rifle with the barrel perfectly level and fire it across a flat paddock, the bullet will hit the ground at almost precisely the same time as one dropped from your hand.
Furthermore, if you dropped several different sizes and weights of bullets at the same instant, say a 55gn .224, a 130gn .270 and a 500gn .458, you’ll find that these bullets fall at the same speed, despite the wide variance in weight. Thus, all bullets respond to gravity in the same way, and have almost the same rate of fall.
This will no doubt confuse some readers and they’ll raise the question that a low velocity 170gn .30-30 bullet will fall a good 50cm when fired at a target 200 metres away. But when you fire a high-velocity .243 Winchester at the same distance the bullet drops only 15cm. How can this be, they ask, if all bullets fall at the same speed?
Of course, the difference is caused by time of flight – the time it takes the bullet to get to where it’s going. When the factor of time enters into calculations, we can easily understand that the .243 bullet dropped less than the .30- 30 bullet because it took less time to cover the distance, thus allowing less time for gravity to act on it and increase its vertical fall. Actually, if the two bullets had started out at the same velocity they would have dropped the same distance.
At this point it would seem logical to assume that if we want a bullet to drop less over a given distance, all we have to do is give it more velocity. But this is only partly true. There are other factors which enter into the equation,and the law of diminishing returns exacts a severe toll very time.
Let’s look at the .243 with 100gn bullet. According to velocity tables, the factory load leaves the muzzle at 2,960 fps. A handloader can increase this velocity by around 90fps – but only at a price. Assuming that the factory load offers the best balance of normal pressure coupled with high velocity, the only way to increase the velocity is by raising the breech pressure. Penalties incurred are more heat, faster barrel erosion, and shorter case life.
But there is a less obvious price to be paid; the faster a bullet starts out the faster it slows down, due to increased air resistance. It is a fact that if you double the velocity of a bullet, the air resistance is not just doubled, but quadrupled! It was Newton who calculated that air resistance is equal to the square of the velocity, but it has since been found that at certain velocity levels it increases even faster.
So let’s look at that .243 bullet again. The pointed bullet starting out at 2,960 fps is still travelling at 2,683 fps at 100 metres, whereas the same bullet starting at 3,050fps remaining velocity of 2,768 fps at 100 metres. The 90 fps advantage at the muzzle has been reduced to 85 at 100 metres, 80 fps at 200 and 74 at 300 metres. If the bullet had been a roundnose rather than a spitzer, the velocity advantage would have been reduced even more.
However, we can overcome the air resistance problem by utilizing the ballistic advantages of streamlined bullets such as spitzer boattails which retain velocity better which also lessens the effect of wind.
Another way to achieve a shorter time of flight – and flatter trajectory – is to use lighter bullets. Factory loaded 80gn bullets in the .243 leave the muzzle at 3,350 fps. They shoot a bit flatter over 300 metres, but the velocity loss is 432 fps over the first 100 metres and 1,185fps over 300 metres.
When trying to achieve a flatter trajectory simply by using a lighter, faster bullet, it is also important to consider the maximum distances you might be shooting. This is because the slower, heavier 100gn bullet will catch up to the lighter 80gn one at 200 metres (2,675 against 2,525 fps) and be travelling faster beyond. At 300 the 100gn bullet actually shows an edge of 334 fps!
Want to go even lighter and faster? Factory loaded 55gn bullets in the 243 streak out of the muzzle at 3,910fps. They have a super-flat trajectory for the first few hundred metres,but the velocity loss is tremendous – 454 fps over the first 100 metres, and a whopping 1,236fps over 300. So save that one for varmints.
To give some idea of how much effect gravity has on a bullet, let’s look at the .243 again. If you hold the rifle with the barrel level when you fire it, the 100gn round-nose leaving the muzzle at 2,960fps will have dropped 67mm at 100 metres, 302mm at 200 metres, and some 772mm at 300. At this point it is curving downward so sharply that it falls another 812mm in the next 100 metres.
With the barrel dead level, a 130gn bullet out of a .270 Winchester drops about 900mm by the time it goes 400 metres. A sleek 175gn spitzer out of a 7mm Rem Mag, falls about double this in reaching out 500 metres. A highvelocity .22 Long Rifle bullet drops nearly 300mm in 100 metres and some 1220mm in 200 metres.
Gravity pulls the bullet down, but its effect can be circumvented. We simply angle the bullet up a little and then let gravity work for us to bring it back down to where we are falling 60mm at 200 metres. Instead the rifle can be sighted-in to zero and hit the target dead-centre at that range. The bullet will land a bit above where we are aiming at shorter ranges. At 100 metres for example, will land 54mm above the point of aim. But this is nothing to worry about. I sight my big-game rifles so that the bullet never lands more than 76mm above the point of aim and this way I know the bullet will hit within 76mm of where I’m aiming all the way out to where it falls 76mm below the point of aim. This distance which will vary according to the cartridge, is the point blank range. Any hit within the 15cm total vertical span will be well placed on most big game. With varmints I sight in 38mm to 50mm high at 100 metres for a 8cm span.
A scope can help the hunter get around gravity because the line of sight is raised 1.5 to 1.8.cm higher above the bore than it is with iron
sights. hit higher in respect to the point of aim at 100 metres than when iron sights are used. This has the effect of making the bullet hit higher in respect to the point of aim at 100 metres than when iron sights are used.
There’s some argument about what mid-range trajectory actually is. Some declare that it’s the elevation of the bullet above line of sight halfway between the muzzle and the point where the bullet’s path crosses the line of sight. But it is really the maximum height between the bullet path and line of sight, no matter at what range it occurs.
The curvature of the bullet’s path changes when it is fired uphill or downhill. Consequently, either way, you need to aim lower, just how much depends on the velocity and the steepness of the slope. With the fast-stepping 130gn/.270 bullet, the point of impact will be about 64mm higher than normal at 200 metres when shooting up or downhill at a 45-degree angle. With a slower bullet such as the 30-30, the point of impact will be nearer to 100mm above where you aim.
This is because the trajectory of a bullet becomes less curved as the up or down angle is increased.
This article was first published in Sporting Shooter, April 2009.