Target Sports

TARGET RIFLE BASICS: ABSOLUTE ZERO

Created on 19th May 2009

CHRIS WHITE says zeroing may not be as easy as you think

MOST RIFLE shooters have some understanding of the concept of zeroing so generally expect it to be straightforward. In terms of establishing an elevation zero it is; wind zero, however, is more complicated. We are into territory abounding with old wives' tales, misconceptions and downright hokum. Indeed the term ‘zero', in respect of elevation, is itself redundant.
So let's start with elevation. Most experienced TR shooters will have a clear understanding of what they mean by their ‘zero' in respect of elevation - they know what elevation to set on their rifle for a given type of ammunition at any particular distance. It is how they get to that point which we need to consider here.

Let's look at this in a bit of a historical context with a fictitious anecdote. It is March 1958 and Jack has just bought a pristine No.4 rifle complete with a brand new AJ Parker rearsight. He turns up at his local range for a pre-season practice session at 200yd and buys a batch of DAC'43 ammunition which he knows is about as good as .303 comes. He lines the zero mark on the rearsight up with 200yd on the range scale and fires a shot that strikes the bottom of the target. He adjusts his sights accordingly and by trial and error ends up with a group of shots, the centre of which coincides with the centre of the bull. When he is finished he loosens the screw on the scale plate and moves it until the zero line lines up with 200yd on the range scale. Now he knows that when he ventures to 1,000yd he will only have to line his zero point up with 1,000 yards on the range scale and he will hit the bull, providing he has judged the wind correctly. Dead simple, one might say. Jack then goes to a few local shoots. He sets his rearsight at the requisite point for the distance he is shooting and finds that, depending on the batch of ammunition with which he is issued, his sighters are generally in the black and he can quickly get into the bull. He gives little thought to the fact that he rarely gets a bull with his first sighter.

Come May he is brimming with confidence and takes himself off to Bisley to shoot in a big open meeting where he is issued with FN ammunition. His sight settings are near enough at 200 and 300yd but setting them at 500yd produces a shot two minutes high and 600yd almost three minutes high. When he repairs to 900yd his first sighter results in the target being pulled down and returned clear with a miss being signalled.

Now the moral of this story is that the range scale has been graduated for ammunition with specific ballistics in an ideal rifle. Not all ammunition behaves the same and neither do all rifles. If Jack had paid attention to the other side of his elevation scale he would have seen that when he set his scale at 200yd, clever Miss Parker arranged things so his sight was reading 5MOA. There is a point to this that we will come back to but just before we leave range scales we need to understand that if we rely on them we are likely to be in a bigger mess than Jack was. If you are shooting a converted P'14 or No.4 your range scale will be gradated for .303 and not the 7.62mm you are shooting!

Even if your rearsight was intended for use on a 7.62mm rifle the chances are it was graduated for British NATO spec. 144gn ammunition, not the 155gn Palma Match ammunition you are likely to be shooting now. Your barrel length may be different from that intended (altering the ballistics of the ammunition anyway), as may be your sight base.

Ignore the range scale

Other than his reliance on the range scale Jack's procedure was basically sound. Let's fast forward 30 years to March 1988. Jack's older and wiser now and he has just bought a new Swing Target Rifle with a Higginbottom rearsight. This has no range scale. The distance of 200yd has become moribund as far as most TR shooters are concerned. Jack takes his rifle onto the range and ‘zeroes' it at 300yd, centring his group on the bull and setting the elevation scale to read zero.

Consulting his AJ Parker elevation chart, Jack now knows his elevation settings at 500, 600 900 and 1,000yd are 6½, 10½, 26½ and 34 respectively. However, he has learned his lessons of 1958 and now keeps a careful record of his actual elevations. From this he knows that his elevations for RG ammunition are close to these figures but for ‘Red Box Norma' he is able to calculate that his rise from 300 to 500 is actually 7MOA, from 500 to 600 4¼MOA, from 600 to 900 18MOA and from 900 to 1,000 7½MOA. So his normal elevations for this ammunition are 7MOA, 11¼MOA, 29½MOA and 37MOA respectively.

Now Jack has done everything as he should - with one exception. He has failed to recognise why Miss Parker thoughtfully graduated his old No.4 sight so it read not zero but 5MOA at 200yd. He discovers this when he travels to Horsford to shoot in the Gresham's School Open and finds himself shooting at 200yd. His sight setting looks like Figure 1. Whether you can read it or not, it is neither easy nor desirable to end up with a negative setting on your rearsight.

Here is how I do it. I try to shoot at the shortest practical distance I can get access to. Usually this is 300yd but it may be 200yd. I shoot a number of shots to let myself and the rifle settle down and then I shoot a group and centre it on the bull. I then set my elevation scale to a figure which ensures that it will always read to a positive value. If I am at 200yd this will be at least 5MOA and if I am at 300yd it will be at least 10MOA. I record that setting and then shoot as many different types of ammunition as I can, recording the mean elevations. The result will look like Figure 2. (See Target Sports January 2006 p.27-28 and February 2006 p.34-35.) This gives a starting point. Settings for each distance are based initially on the rises from 300 (Table 1) and then confirmed by recording corrected elevations from actual shoots.

That's all there is to it - if we have a fixed foresight. If we have an ‘up-and-down' foresight, the procedure is a little more involved. We'll deal with that next time.

TARGET RIFLE BASICS: ABSOLUTE ZERO

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TARGET RIFLE BASICS: ABSOLUTE ZERO

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