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Grain
Measure of weight applied to pellet. 7,000 grains = 1 pound. 1 gram = 15.43 grains.
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Groove
The low point of rifling within a barrel
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Groove Diameter
The diametrical measurement of the bore of a rifled barrel, measured from the bottoms of opposing grooves (i.e. the largest internal dimension). If the grooves are not opposed, the diameter of a circle inscribed to touch the bottoms of the grooves is taken. This measurement should be fractually larger than the true diameter of the appropriate pellet.
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Group
A cluster of pellet holes made by the same rifle/pellet combination, formed from numerous shots fired at a target using the same point of aim, for checking accuracy. A 5-shot group of 1 inch at your shooting range eg 45 yards (measured from the centres of the two widest spaced holes) is generally regarded as acceptable airgun hunting accuracy. It is a statistical fact that group size will increase with the number of shots fired.
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Group Sizes
It is the maximum distance between the centres of the two farthest shots in a group. The easiest way to do this is to measure from the outside edge of one pellet hole to the inside edge of the farthest one away. Another method is to measure the distance from outside edge to outside edge of the farthest apart holes, then subtract the pellet hole diameter. ( Note that the pellet hole diameter is often smaller than pellet diameter - check it for yourself! ) This latter method allows recording groups smaller than the pellets diameter itself.
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