The end of the barrel in contact with the cartridge case is called the "bore", which is used to locate the position of the bullet;
The rifling part of the barrel is called rifling, which guides the bullet to rotate;
The joint between the bore and the line bore is called "slope bore" throat, which guides the warhead to be embedded in the rifling, and it is also the place with the fastest ablation and wear;
In western rifles, a cylindrical section will be connected in front of the bore, and the gas will be sealed before the warhead is embedded in the rifling, which is called "lead";
At the muzzle, in order to ensure the uniform discharge of gas at the moment when the warhead is discharged, the muzzle chamfer should be processed. The general automatic rifle is 1.5x45, and the sniper rifle is much larger, so that if you accidentally touch the muzzle during use, it will not damage the muzzle chamfer and the stability of the muzzle gas field.
These are gun bores, which are an important part of the barrel.
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The length of the barrel of M4 is 368mm, but the depth of the barrel is not 45mm, because the bullet is not completely loaded into the barrel, but the tail is exposed outside the barrel. If you subtract the depth of the socket at the bottom of the bolt, you can't take out the shell, and the depth of the gun bore is only about 42 mm.
368mm refers to the size of the barrel as an independent part, that is, when the factory produces the barrel, the finished product is 368mm, including the bore depth of 42 mm, of course. If this 368mm has no bore, does the factory have to weld another piece at the back? ... people with a little knowledge of machining wouldn't do that.