4. Definition of choke – on the target

Density of shot on the target

The second way of defining choke is to consider the nature of the pattern thrown by a given barrel and ammunition combination. If the barrel throws 70% of the shot inside a 30 inch circle at a measured 40 yards from the shooter, then it throws full choke patterns and hence can be considered a full choke barrel regardless of the actual boring. (It is a full choke barrel with that ammunition, and may well behave differently with different ammunition.) The definitions of choke according to the patterns thrown are given in the tables below.

Table 7: Percentage patterns for all borings at all distances in yards (source: GT)

Table 8: Percentage patterns for all borings at all distances in metres (source: Eley)

NOTE: All shotguns of a given choke specification – regardless of gauge – should throw the same percentage of shot inside the 30 inch circle and have the same diameter of pattern. The total number of pellets will vary according to gauge and load, but not the percentage of shot nor diameter of pattern.

Distribution of shot across the pattern

One of the quirks of a heavily choked barrel is that, not only is the overall diameter reduced compared to less choke, but more choke tends to concentrate the shot density to the centre of the pattern rather than merely uniformly across a smaller diameter. This can be illustrated by dividing the 30 inch circle into two concentric circles, one 30 inch diameter and the other 20 inch diameter. The 20 inch circle occupies an area which is only 45% that of the 30 inch circle. Given a uniform distribution of shot, therefore, the inner circle should be struck by 45% as many shot as strike the total area inside the 30 inch circle. Does this happen in reality? No. It has been found that as the degree of choke increases the degree of concentration inside the 20 inch circle increased disproportionately (although the effect lessens with distance.)

Table 9: Percentage of total shot strikes inside a 20 inch circle (source: GT)

The implications of this are that a bird hit squarely in the centre of the pattern from a full choked barrel at medium range (e.g. 30 yards) is likely to be struck by an excessive number of shot, thus incurring the wrath of the cook. It also means that more birds will be missed at close to medium ranges.

Diameter of pattern

The diameter of the pattern is another measure of performance. In short, the wider the diameter produced by a barrel, the less accurate the shooter has to be (provided the density is adequate – and therein lies the compromise). The narrower the pattern, the more accurate we must be. So too much choke is a bad thing for most forms of shooting in Britain. Table 10 and Table 11 below give the pattern diameter that is covered by the bulk of the shot charge at various distances according to the stated degree of choke of the barrel.

Table 10: Diameter of spread in centimetres for distances in metres (source: Eley)

Table 11: Diameter of spread in inches for distances in yards (GT)

The reason for many ‘unexplained’ misses at close to medium distances with a heavily choke barrel is obvious from the above. One only needs to compare the cylinder boring at 20-30 yards to the full choke at 35-40 yards. The heavily choked barrel is only really suitable for shooting where most of the shots are taken at 40+ yards.

Patchiness of pattern

A typical pattern produced on the flat patterning plate will include some areas through which a bird can ‘escape’. Many people consider that an empty 5 inch disc represents a ‘missed bird’ (but see later). GT reported that the number of non-overlapping 5 inch circles (the vital area of our pheasant) in which there were no shot strikes on a normal 12 gauge 30 inch pattern were as follows.

Table 12: Non-overlapping empty 5 inch circular patches on typical patterns

Of course, the pattern plate is 2-dimensional but the shot pattern in the air is 3-dimensional, and shaped like a large sausage or melon. Approximately half the shot is in the front third of the sausage. The 5-inch vacant patch represents a ‘tube’ along the length of the pattern. Unless our bird is flying directly up that tube, he will be hit by several shot – so the reality is better than the pattern plate might have us believe. J Some manufacturers measure the quality of their patterns only by the presence or absence of the 5-inch vacant discs.

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