AIMING is one of rifle shooting's essential skills, but there is more to it than first meets the eye.
Sighting-in is part of the deal, because the rifle must be zeroed at a specific distance, chosen to suit the bullet's trajectory. That is, we must know how far the bullet will rise above and fall below our line of sight, and at what distances.
In paddocks and scrub, we have to estimate the target's range, and then decide whether we need to aim high or low, in order to compensate for the bullet's trajectory. We also have to allow for the effects of wind and mirage or heat haze.
Trigger squeeze must be gentle and controlled, to avoid pulling the rifle off line.
Then there's the shooting position. Experienced shooters will not aim offhand (that is, standing up, with the rifle supported only by their hands) when they can kneel, sit or lie down. If some extra support is available, such as a rifle sling, a bipod, a tree branch or a log, they will use it.
In all positions, the secret is to ensure that the rifle lines up with the target with a minimum of muscular effort.
If aiming calls for lots of muscle work, one's wobbles and twitches get worse. If you have to muscle the rifle to one side in order to keep it lined up, the shot is more likely to fly wide.
So most of the aiming should come from the placement of torso, legs, feet and arms. In other words, aim the rifle with the whole of your body. Shuffle around until the vertical crosshair, blade or bead lines up correctly, leaving only small elevation adjustments to be made.
Now let's return to canting, the subject of our last column, (WT, June 9). Our focus was field rifle shooting.
Canting means allowing a rifle to tilt to one side while we are aiming it. This causes the shot to fly low and wide, in the direction of the tilt.
The maths are simple.
All we need are the cosines and sines of the cant angles, plus drop values. The former are found in maths sources, while drop figures can be obtained from exterior ballistics tables or the ballistics programs available for personal computers.
Use these formulas, where E is the elevation error, W is the windage or sideways error, A is the cant angle and d is the drop:
E = d(1 - cos A). On a calculator, this is "1 minus the cosine of A, multiplied by the drop".
W = d x sin A. On a calculator, this is "the drop multiplied by the sine of A".
For example, take a .22LR rimfire with a 40 grain solid projectile launched at 1100 fps. At 50 metres, the drop would be 118mm.
A 10-degree cant would then cause the bullet to fly 20mm wide and 1.8mm low. At 100 metres (507mm drop), the same cant angle would yield a shot 88mm wide and 8mm low.
These results follow the usual pattern. That is, the windage error is much greater than the elevation error, and may be big enough to ruin the shot.
Fortunately, we don't have to spend time calculating canting errors. It is much simpler to ensure that canting doesn't happen!