Gas Check - Shallow copper cup attached to the base of appropriate cast bullets to reduce barrel leading and to protect the base of the bullet from the effects of the burning powder gases.

Gas Leak - Black marks around primer showing where gas has escaped.

Gas Operated - An automatic or semiautomatic type firearm in which the propellant gases are used to unlock the breech bolt and then to complete the cycle of extraction and ejection.

Gauge - A term used in the identification of most shotgun bores, but has no relation to the linear measurement of the bore. (410 bore is an exception). Gauge is determined by the number of perfect spheres which may be obtained from one pound of lead. (Example, a 12-gauge gun has a bore diameter the same as one of the 12 perfect spheres which could be made from a pound of lead. If 20 perfect spheres are made from one pound of lead, the diameter of one of these spheres would be the same as the diameter of the 20-gauge gun). The bore diameters of various gauges are as follows: 10 gauge/.775"; 12 gauge/.725"; 16 gauge/.662"; 20 gauge/.615"; 28 gauge/.550"; 410 gauge/.410". (abbr. Ga)

Gilding/Guilding Metal - Cupro-nickel (copper/nickel alloy) or copper-zinc alloy used for bullet jackets.

Girth - The smallest circumferential dimension at the pistol grip of a stock.

Grain - Measure of weight applied to bullets and powder. 7,000 grains = 1 pound. 1 gram = 15,43 grains.

Greenhill’s Formula - A formula that relates bullet weight and length to rifling twist. Having two of them, the third can be calculated. One must just keep in mind that Greenhill's formula assumes cylindrical, pure lead bullets, and doesn't work very as well for small calibres as it does for large bores. L = bullet length in inches D = bullet diameter in inches [150/(L/D)]xD = twist in inches.

Groove - The low point of rifling within a barrel.

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 bullet. See Bore diameter.

Group - A cluster of bullet holes made by the same rifle/load combination, formed from numerous shots fired at a target using the same point of aim, for checking accuracy.  A 5-shot group of 1½ to 2 inches at 100 yards/meters (measured from the centres of the two widest spaced holes) is generally regarded as acceptable hunting accuracy for factory rifles using factory ammunition.  Groups of 1" at 100 yards are regarded as excellent.  For target shooting, especially benchrest, groups under .35" is the norm.  For standardization, it is best to fire five-shot groups with the same aiming point. It is a statistical fact that group size will increase with the number of shots fired.

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 bullet 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 bullet hole diameter. ( Note that the bullet hole diameter is often smaller than bullet diameter - check it for yourself! ) This latter method allows recording groups smaller than the bullet diameter itself. An example is in some benchrest competitions where the winning group with a 6 PPC rifle ( .243-inch bullet ) will be under 0.2 inches.

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