SHOTGUN BORES AND CHOKES - ICSI



SHOTGUN BORES AND CHOKES

The bore diameter in a 12 bore shotgun can range from 0.719 inch to 0.740 inch (18.5mm to 18.7mm). The nominal boring is usually 0.729 inch/18.517mm. The Sporting Arms and Ammunition Manufacturers Institute (SAAMI) give the nominal boring of shotguns for the following bores as 10 bore a boring of 0.775 inch, 12 bore a boring of 0.729 inch, 16 bore a boring of 0.665 inch, 20 bore a boring of 0.615 inch, 28 bore a boring of 0.550 inch and the .410 a boring of .410 inch. The following table gives you the diameter for the range of common bore sizes.

| |NOMINAL SHOTGUN BORE DIAMETERS |

|Bore |10 |12 |16 |20 |28 |.410 |

|Inch |0.775 |0.729 |0.665 |0.615 |0.550 |0.410 |

|mm |19.685 |18.517 |16.891 |15.621 |13.970 |10.414 |

The gauge or bore was originally determined as we all know from the melting of an imperial pound (16 ounces) of lead into spherical balls. This inside diameter is based on the number of pure lead spherical balls each fitting the bore. A 12 bore shotgun would therefore accept one of 12 equally sized spherical balls, which when weighed together would equal one pound. This holds true for all bores, except the .410 which is shown in decimals of an inch as .410. The bore diameter is measured 9 inches from the face of the breech.

The concept of choke is claimed to have been invented by an Illinois duck hunter Fred Kimble in 1867, according to our American shooting cousins. It was also an American gunsmith Sylvester Roper who patented a system of screw-in chokes in 1866. However as we all know it was an English gunsmith William Rochester Pape of Newcastle who was granted British Patent No 1501 on the 29th May 1866, in which the use of choke was described. This was further trialled and improved upon by another great English gunsmith W.W. Greener in 1874. It should also be noted that the merits of choke were being discussed as early as 1827. However since shotguns and the blunderbuss with its bell shaped muzzle had been used since the first firearms were invented; it is likely that an understanding of choke predates all of these.

Choke size is relative to the diameter of the bore and the constriction of the muzzle and is determined by subtracting the diameter of the constriction from that of the bore. Bore diameter is measured 9 inches (223mm) from the breech with either bore plug gauges or a specially adapted micrometer dial gauge, which a gunsmith or good quality gun shop would use. The use of a brass choke gauge as sold in most gun shops and owned by a lot of shooters may give you a rough idea of the guns choking but does not give you a true indication of the actual choke which you would get when used with the appropriate gunsmiths gauges. Choke is measured in thousandths of an inch and several thousandths of an inch can easily affect the pattern of a cartridge. An open choked gun will offer a wider pattern of shot compared to a tighter choke which will offer a restricted or tighter pattern which is slower to spread and is still fairly dense at 40 yards. Pressure and the consistency of the lead will also affect patterns, as can altitude. Pattern and shot stringing formation depends on how powerful the rear wad pressure is. If it is heavy as in the case of an improved cylinder or ‘open’ choke the charge is virtually sandwiched between the opposing forces of wad and air resistance and the pellets spread outwards, widening the pattern. In a full choke the wad is slowed noticeably. The choke retards the wad and the pellet string narrows down to squeeze through the narrow opening. Since the wad is delayed by the choke taper the pellets continue on a tighter, straighter course because they are not being rammed from behind, as in the case of an open choke. Choke then, is important only as it retards or fails to retard the wad, and in how it prepares the shot charge for its eventual impact with air resistance.

A high velocity and normal soft lead cartridge will give a wider pattern and a low velocity and normal soft lead cartridge will give a tighter pattern. Better patterns can be achieved with normal soft lead and heavier sized shot cartridge. It is strongly considered that the general or ideal choking of a shotgun should be improved cylinder and ¼ choke. However there are some theories that the tighter the choke the better the pattern over the chance of a chipped clay or wounded quarry is eliminated, but often a gun can be ‘over-choked’, resulting in the shooter having to be more precise and accurate on a target; something the average shooter may not be. Patterns also become elongated into a “shot string”. This elongation is approximately 10 – 12 feet at 40 yards. There is a common misconception in the understanding of shot stringing in relation to choke. Many believe, and “common sense” appears to support the notion that tighter chokes produce longer shot strings; however, tests have proven that the opposite is true. An open choke causes a longer “shot string” and a tighter choke causes a shorter “shot string”. In simple terms, the constriction of a tight choke crams the pellets into a smaller mass, which stays together longer after it leaves the muzzle, thus reducing the “stringing” effect.

[pic]

Diagram reproduced with kind permission of the CPSA

If we take an average shotgun with a bore diameter of 0.729 inch and the restriction measurement at the muzzle is measured at 0.709 inch, the difference is measured at 20 thousandths of an inch. This equates to a choke of ½ choke. Similarly a gun with a bore diameter of 0.740 inch and the restriction measurement at the muzzle is measured at 0.720 inch, the difference is again measured at 20 thousandths of an inch. This would also equate to a choke of ½ choke. Taking this information and firing the shotgun at a pattern plate/sheet at a distance of 40 yards the pellet count in a 30 inch circle can also give you the choke of the gun, but takes a little more time when having to count the actual pellets. See the table below for comparisons of choke, restriction measurements and percentages of pellets at a distance of 40 yards.

|CHOKE |CONSTRICTION |PERCENTAGE OF PELLETS |

|Full |0.040” |1.016mm |70% |

|¾ |0.030” |0.762mm |65% |

|½ |0.020” |0.508mm |60% |

|¼ |0.010” |0.254mm |55% |

|Skeet |0.005” |0.127mm |50% |

|Cylinder |0.0” |0.0mm |40% |

Andy Kirkland FICSI

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