8 bore
Encyclopedia
Eight bore, 8 bore, or 8 gauge is an obsolete caliber
Caliber
In guns including firearms, caliber or calibre is the approximate internal diameter of the barrel in relation to the diameter of the projectile used in it....

 used commonly in the 19th century black powder firearms. It may refer to either rifles or shotguns. It is the size between the larger four bore
4 bore
Four bore or 4 bore is an obsolete black powder caliber of the 19th century, used for the hunting of large and potentially dangerous game animals. The specifications place this caliber between the larger two bore and the lesser six bore...

 and the smaller ten bore. In modern times, this size of shotgun shell has a use in cleaning kilns, by blasting away deposits from a distance, this can be done while the kiln is in operation in some instances. Various loadings are in production for different industrial uses.
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