Degree of shot dispersion

The degree of dispersion of the shot is dependent on many factors, the most important of which are:

• cartridge pressure;

Cartridge type. The higher the pressure generated by the cartridge, the more the shot will be disrupted as it emerges from the barrel by the following gases. Whilst this effect is largely offset by the wads used in modern cartridges, it is a factor which does affect the dispersion of the shot.

Wad type. The wadding in traditional shotgun cartridges consisted of an over-powder wad, a series of filler wads and an overshot wad.

The over-powder wad was intended to act as a gas seal to prevent the high-pressure gases from escaping up into the shot column during firing. The filler wads were to cushion the shot against the rapid acceleration during its progress down the barrel, and the over-shot wad was to retain the shot in the cartridge case.

This construction did, however, suffer from a number of drawbacks. On firing, hot gases could escape passing the over-powder and filler wads into the shot charge. This could result in pellets becoming partially melted and fused together. Likewise, the filler wads did not provide sufficient cushioning of the shot to prevent distortion due to inter-shot contact or, in extreme cases, cold-welding together small clumps of shot due to pressures produced.

Also, during the passage of the shot through the barrel, severe distortion could occur through contact with the inside surface of the bore. Shot which had become so distorted would not fly in a predictable manner resulting in distorted and enlarged patterns.

Modern wads tend to be of the plastic cup type with an integral shock absorber and gas seal. The shock absorber consists of a semi-collapsible section which very effectively cushions the shot column at the moment of acceleration. The integral plastic cup protects the shot during its passage through the bore, and the plastic gas seal prevents the leakage of hot gases into the shot column.

Cartridges loaded with this type of one- piece wad (mono wad) will give a much more controlled spread of shot than one loaded with the old-type wad column (Figures 5.16-5.17).

Case example: cold-welding of shot. An example of how shot, unprotected by a modern cup- type mono- wad system, can become cold- welded involved the jilted boyfriend of a girl who was a very keen horse rider. The ex-boyfriend was so upset that he decided to teach the girl a lesson. Hiding in a clump of trees close to a bridle path where the girl normally went riding, the ex-boy friend lay in wait with a 0.410" shotgun loaded with very fine (dust) shot. His plan was to fire at the horse, which would feel the prick of the dust shot, and bolt, thus 'teaching the girl a lesson'. Unfortunately, the dust shot cold-welded and came out of the barrel as a solid lump and instead of hitting the horse went straight through the neck of the girl, virtually decapitating her.

Barrel choke. Except for a few very unusual weapons, shotguns have smooth-bored barrels. Most sporting weapons have a constriction applied to the muzzle

A. Overcharge card wad

B. Undershot wad

C. Overshot wad

D. Cushion wads E Shot column

F. Rollover crimp

G. Propellant

H. Base wad

Overshot Wad

Figure 5.16 Shotgun cartridge.

Choke Bore Size
Figure 5.17 Modern shotgun cartridge loaded with a mono wad.

end of the weapon's bore to control the spread of the shot. This constriction is called 'choke'.

As the column of shot passes through this choked part of the barrel, the diameter of the shot column is reduced, thus elongating its length. This reduction in diameter results in the outer layers of shot in the column being given an inward acceleration. This inward acceleration delays the spreading of the shot once it leaves the barrel, thus reducing its degree of dispersion.

The tighter the degree of restriction, the tighter the pattern of shot at the target. The usual degrees of choke are called full, three-quarters, half, quarter, improved cylinder and true cylinder. The degree of constriction is irrespective of the bore of the weapon; thus, all full-choked barrels have a constriction of 0.004", all half-choked barrels 0.002", improved cylinder 0.0005" and true cylinder no restriction at all.

One important thing to notice from these figures is that all bores should give the same spread of shot with a given 'choke' at a given distance. The only variation being that as there is less shot in a 0.410" bore cartridge than a 12-bore cartridge, the density of shot at the target will be less.

The degree of choke is based upon the percentage of the total pellets in a cartridge which will be in a 30" circle at any given range.

The following tables (Tables 5.1 and 5.2) show the effect of choke on pattern measured by percentage of shot in a 30" circle and the spread of shot at various ranges for cartridges loaded with mono wads.

The spread of shot is the diameter of a circle which contains the majority of the shot charge at any given range. Factors such as irregularly shaped shot, shot which has contacted irregularities in the bore, pellets which become embedded in the wads, and so on, can all give rise to what are called 'flyers'. These flyers are pellets which do not fly true with the rest of the charge, falling outside the main body of shot at the target. Deciding which pellets constitute flyers and should thus be excluded from the main spread requires much experience with the examination of shot patterns and is something which just cannot be explained in a book (Figure 5.18).

Benelli Raffaell Crio Choke Ret
Figure 5.18 Example of a shotgun pellet pattern showing flyers.

Table 5.1 Showing the percentage of total pellets in a 30" circle for various chokes.

Range in yards

Range in yards

Table 5.1 Showing the percentage of total pellets in a 30" circle for various chokes.

Barrel choke

20

25

30

35

40

45

50

55

True cylinder

80

69

60

49

40

33

27

22

Improved

92

82

72

60

50

41

33

27

Quarter

100

87

77

65

55

46

38

30

Half

100

94

83

71

60

50

41

33

Three-quarters

100

100

91

77

65

55

46

37

Full

100

100

100

84

70

59

49

40

Table 5.2 Showing spread of shot, in inches, for various chokes.

Range in yards

Table 5.2 Showing spread of shot, in inches, for various chokes.

Range in yards

Barrel choke

10

15

20

25

30

35

40

True cylinder

20

26

32

38

44

51

58

Improved

15

20

26

32

38

44

51

Quarter

13

18

23

29

35

41

48

Half

12

16

21

26

32

38

45

Three-quarters

10

14

18

23

29

35

42

Full

9

12

16

21

27

33

40

Using the above table and the one following, it is possible to determine the actual number of pellets in a 30" circle in any of the six borings of a gun at the stated ranges (Table 5.3).

Example: Number of pellets striking within a 30" circle at 40 yd for a y choked barrel with a charge of 1-1 oz no. 6 shot. Total pellets in 1-1 oz shot is 287. This is multiplied by 60 (% in circle at 40 yd) divided by 100 = 172.

Barrel length. Shortening the barrel by sawing off the muzzle end does have some effect on the spread of shot. It is, however, not as great as popular tradition has it.

What effect exists, however, is mainly due to the high-pressure gases disrupting the shot column as it exits from the barrel.

Shotgun propellants are very fast-burning, giving rise to a very sharp rise in pressure during the first few moments of ignition. In full "length barrels, the overall pressure within the barrel drops very considerably as the shot nears the muzzle and the volume of gas between the over-powder wad and the standing breech of the weapon increases.

As the barrel is progressively shortened, the pressure being exerted on the base of the shot column as it exits the barrel becomes progressively greater.

Table 5.3 Actual number of pellets stoking within a 30" circle.

Number of pellets in shot load

Shot size

Shot size

Table 5.3 Actual number of pellets stoking within a 30" circle.

Number of pellets in shot load

Weight of shot (oz)

3

4

5

6

7

8

If

228

276

358

439

552

732

H

210

255

330

405

510

675

4

175

213

275

338

425

562

ii6

160

202

261

321

404

534

157

191

248

304

383

506

149

181

234

287

361

478

i

140

170

220

270

340

450

16

131

159

206

253

319

422

8

122

149

193

236

298

394

13 16

113

138

179

219

276

366

5 8

87

106

138

169

212

282

9 16

78

96

124

152

191

254

7 16

61

75

97

118

149

187

5 16

44

53

69

84

106

141

These pressures can lead to a destabilization of the shot column and a 'blown' pattern. This effect can be identified by an irregular shot pattern and a larger spread than would normally be expected.

The effect is much less than popular tradition would have us believe and is only marginally greater than that for a full-length cylinder bored barrel.

5.5.3 Summary

As can be seen from the above, the discharge residue exiting from the muzzle of the weapon can provide extremely useful information in respect to the range of firing. At close ranges, up to 2 ft (60 cm) in handguns and 6 ft (2 m) in rifles, estimations can be made as to the range of firing. If the weapon and ammunition type are known, an accuracy of + or - 2 in. (5 cm) can be obtained.

This is normally carried out by direct visual examination of the target material. On heavily bloodstained or dark clothing which might camouflage the presence of the discharge residues, IR photography or chemical methods can be used to visualize the residues.

At greater ranges, the SEM can detect sub -microscopic particles of primer residues which can give range estimations up to 6 or more feet (2 m), but with a much lower accuracy.

With shotgun ammunition, the spread of shot can provide range of firing estimations with reasonable accuracy up to the maximum range.

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Responses

  • sean
    How to measure "degree of restriction" of gun?
    8 years ago
  • filmon
    How is pressure out of gun barrel dispersed?
    3 years ago
  • bruce williams
    Why choking reduces dispersion of pellets?
    2 years ago
  • gebre
    What is the degree of spread from a shotgun?
    9 months ago
  • Jamie-leigh
    How many pellets do i need in a 30 inch circle?
    5 months ago
  • yvonne
    How to limit rotational dispersion in rifled shotshell loads?
    2 months ago

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