I would say either the 5.56mm or the 7.62mm.
The caliber of a bullet, refers to it's diameter in inches. But sometimes other measurements are used, such as mm.
There are more people killed each year from a 22lr than any other gun!! There are more people killed each year from a 22lr than any other gun!!
that depends on caliber, bullet weight, powder type, amount of powder, the gun used and other factors.
It depends. A box of 10 .50 BMG cartridges are something like $25 or $30. .50 AE is much cheaper. There are also other types of 50 caliber ammo and bullets.
.38 Special, yes. Other .38 cartridges (such as the .38 Long Colt or .38 Super), no.
38 is the size. This answer is actually correct, but more specifically, .38 special (and many of the other .38 caliber cartridges) are actually .357. The caliber ".38" was chosen to distinguish between .357 magnum and .38 special.
Caliber refers to the diameter of the bullet. A 50 caliber bullet is .50 of an inch (half an inch) in diameter. It's also used to refer to a gun that shoots a 50 caliber bullet. The problem with the word "caliber" is that it has two different and confusing meanings. In small arms, "caliber" usually refers to the diameter of the bullet, NOT the casing, and is measured in inches (i.e. .50 caliber is 50/100ths of an inch, or a half-inch). [Note: your original question should be ".50" caliber, not "50" caliber.] The biggest problem with this usage is that it's not exact - the most common .50 caliber bullet (the .50 BMG of the Browning .50-caliber Heavy Machinegun) is actually .510 inches at the widest point. The other use of the word caliber is the ratio of the barrel bore (inside) diameter to its length. For instance, the huge 16" guns on the US's Iowa-class battleships are properly classified as 16"/50-caliber guns, which means that the barrel length is 15 times the bore length; thus, the 16/50 is 800 inches long (66 feet, 8 inches). While this usage of caliber is most common for cannon (and not small arms), it nonetheless can be found in discussions of rifles.
The most basic answer is, a 9mm has a smaller diameter. Other differences, (in most cases) less muzzle energy, smaller and lighter bullet, higher velocity.
Caliber is used to relate the diameter of a projectile (bullet). Its possible for two projectiles to have the same diameter, but vastly different weights/masses. One bullet might be 3 times as long as the other, although both have the same diameter. Caliber is typically measured as hundredths of an inch. So a 38 Caliber implies a projectile with a diameter of 38/100th of an inch, or .38 Cal. Notice the leading decimal point in most caliber designations. .45ACP is 45/100th of an inch. ACP is a special, patented shape of round designed by the Colt company for its automatic pistols. ACP = Automatic Colt Pistol.
The short answer is no. The longer answer is, you might be able to get a .40 caliber cartridge to fire from a .45 caliber gun, but it may damage the gun, and even if it doesn't damage the gun, it will cause other problems. In general (with only a few exceptions) you should never try to fire any cartridge in any gun other than a gun designed for that caliber.
People are not killed by guns. People are killed by other people. Some use guns, some use other objects.
It can only safely accept the 270 Weatherby Magnum if that is what it is chambered for. If you want to chamber it in some other caliber than get a new rifle. Just because two bullets have the same diameter bullet (270 or any other caliber) does not make it safe to fire it in your gun.