text: Unfortunately, explanations of caliber and bore can differ, depending upon your particular purpose (even within the context of firearms).\n\n   Caliber, as a standard of measure, is given in tenths of an inch, though for technical purposes, thousandths of an inch are also given. Some will state that this later case is not correct.\n\n Example:\n   an M1 Garand fires a bullet having a diameter of .308"\n   an AK-47 fires a bullet having a diameter of .311"\n   They are both referred to as being .30 caliber rifles.\n\n   The bore of the gun is the inside of the barrel (like a "hole," it's not actually anything concrete, but where stuff is missing) it can also refer to the inside walls of the barrel ("there is rust in the bore")\n\n   Most barrels have rifling, which is also described as "Lands and grooves" the lands are the areas sticking up, and the groves are the areas pressed (or cut) away.\n   If you imagine a cross-section of a gun barrel now, you would see something like a circle with a "gear tooth" edge. (remember that the metal is on the outside of this shape, not inside it) You could draw one circle touching the tops of the lands, following the inside shape (cutting off the teeth), and one circle touching the bottoms of the grooves (making the circle as big as the teeth).\n   The diameter of the smaller circle (measured from the tops of the lands) is the Bore Diameter. It is the largest round thing you could pass through the barrel, without having to miss-shape it to make it fit.\n   The diameter of the larger circle (taken from the bottoms of the groves) is the Groove Diameter. This represents pretty much the largest thing you can pass through the barrel, and whatever you push through it, will be cut, or pressed into by the lands of the barrel sticking up, in its way. Bullets, for the most part, are this diameter.\n   Caliber USUALLY refers to this larger size, and thus, too the diameter of the bullets used in the barrel too.\n\n   To give an example (I think these numbers are correct, but I have not verified them):\n   If you have a .30-06, the bullet 308/1000 of an inch (.308") passes through the barrel, which has a BORE of .300" and a grove diameter of .308".\n   If you have a .50 BMG, the bullet .510" passes through a barrel with a bore of .500", and a groove diameter of .510"\n\n   Unless you are involved in some technical aspect respecting firearms, most people will use the bullet's diameter "Caliber" to describe both the barrel and the bullet, because one number for everything is much easier for everyone involved.\n\n\n   As for bullets and weird sizes, I'll add to that: Why do they use numbers that don't reflect anything accurately?\n   You will find, in many cases, that weapons firing bullets of seemingly similar calibers, even if from different cartridges, are, in fact, the same caliber bullet. Markets usually don't reinvent the wheel. The .30-06, .308, .300 Win Mag, and .30-30 (I believe) all use the same .308" standard. similarly, 6mm and .243 cartridges are using the same caliber, and most center-fire (not rim-fire) .22s are using the .224" bullet. These include the .22-250, .220 Swift, .223 Remington (aka 5.56mm x45/NATO) and more.\n   I've always found it interesting to note that, while it makes sense to round .308" to a nice, round "thirty," that the actual metric equivalent to that is 7.82mm, and that the no-less-complex 7.62 is simply a nice, round "thirty" in inches.\n   Despite what I state here, if you're dealing with foreign cartridges, you will often find them using similar but different actual diameters. (the British .303 is something like .312" and the AK-47 is .311" I believe)\n   Also worth mentioning is that the .38 is a .357" and the .44 is a .429". Sometimes they're not that close.\n   The best advice I can give you