Chapter 3 Shot Cloud Phenomena
Using averaged flight time data for the leading and trailing edges of the shot cloud the longitudinal distribution of pellets is examined to ascertain the effects of altering the launch conditions (such as the internal ballistics of the barrel) and pellet properties.
The lateral positions of the pellets are normalized about the pattern centre and the standard deviation of their x and y pellet co-ordinates are obtained. This gives a measure of the pellet dispersion and is used to examine the relationship between the lateral distribution of pellets and the internal ballistics of the barrel. A more in-depth investigation is then undertaken to determine the true nature of the lateral distribution of the pellets. It is then possible to examine the appropriateness of the traditional approximation (pellet count within a 30" circle at 40 yards) that the average lateral pellet density distribution is a radial Gaussian. To conclude the analysis on the average lateral distribution of pellets, the Poisson process is shown to be an appropriate approximation for describing the hit probability distribution within small areas of the pattern.
p. 98 It was found that the typical percentage variation in the number of pellets was 1 % for mass produced cartridges. This variation in the number of pellets along with the variation in pellet mass, which is controlled by the pellet diameter, affects the load weight of the cartridge and was seen to vary by 1%.
Powder variation for commercial lead cartridges was 0.7%; steel 0.9%.
There is a percentage variation of 2.65% in the diameter and a visible variation in the shape of the lead pellets.
p. 99-100 Pellet deformation of #4 lead, steel (very little), zinc & bismuth (fractured) pictured.
p. 102 The effect of choke on the deformation of the fired pellet was studied. With the exception of steel pellets, it was found that the greater the constriction, the worse their shape became.
The variation in the wads had a great effect on the patterns because if the wad does not release the pellets consistently then the initial spread is altered, and therefore so is the final pattern. It has been observed that the wads do not open in the same way and fall away to different parts of the test range. They have been seen to travel with the shot for a long period of time (seen in the shot cloud profile and by the skyscreens), spin off at strange trajectories (visually), possibly spilling the pellets, or fall away quickly leaving the pellets to travel downrange.
150 different 10 cartridge batches of loads were tested
p. 110 For choke constriction greater than .030 leading edge shot column velocity at the muzzle increased only 2%.
p. 113 The leading edge velocities are given for several different material loads and it can be seen that altering the choke has an effect on the downrange ballistics of the shot cloud. Pellets in a shot cloud produced by the tighter choke spend less time subject to the full force of the air resistance and achieve a 5% greater velocity downrange.
p. 115 The longitudinal distribution of the shot cloud at the measurement ranges 20-50m is controlled more by the variation in aerodynamic performance of the pellets rather than the initial spread produced by the choke.
The shot cloud length behaviour, for any pellet material, at ranges between 20-50m has indicated that it has no relationship with the initial launch condition. However, a trend has been observed between different loads of pellet materials with the same diameter and load weight. The shot cloud lengths are given for a selection of 32g
loads containing either lead, steel, bismuth, or zinc #5 shot shows in general that the steel load produces the shortest length over the measurement range, whereas lead loads are of similar length at 20m but produce a much longer shot cloud at
50m.
From the characteristics of the lead profiles, the pellets are shown to slowly expand in the longitudinal direction with a few impacts breaking away at the trailing edge of the main section at a range above 35m. This associated stringing out effect may be explained by the variation in the in-flight pellet shapes. For steel loads, the shot cloud profiles, are generally more compact over the measurement ranges, but with their greater volume of pellets in flight, the lengths at 20m for the same pellet diameters and load, are seen to be longer than those for lead. However, with their associated smaller variation in steel pellet shape, and therefore in drag, the diffusion rate is less in the longitudinal direction and results in the shortest shot cloud length at 50m.
The conclusions from investigating the effects of choke on the longitudinal distribution of pellets in a shot cloud are that the in-flight effects are dominant at the measurement range of 20-50m.
p. 122 The longitudinal distribution of pellets in a shot cloud is best described by a Rayleigh distribution and the effect of choke on it at the measurement range 20-50m is minimal.
p. 123 Effect of choke (.000 - .050) on lateral pattern spread at 40 yds.
