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.

In a rifled barrel, the distance is measured between opposing lands or grooves; groove measurements are common in cartridge designations originating in the United States, while land measurements are more common elsewhere. It is important to performance that a bullet should closely match the groove diameter of a barrel to ensure a good seal. When the barrel diameter is given in inches, the abbreviation "cal" is used in place of "inches." For example, a small bore rifle with a diameter of 0.22 inch is a .22 cal; however, the decimal point is generally dropped when spoken, making it "twenty-two caliber" or a "two-two caliber". Calibers of firearms can be referred to in millimeters, as in a "caliber of eighty-eight millimeters" (88 mm) or "a hundred and five-millimeter caliber gun" (often abbreviated as "105 mm gun").

While modern cartridges and cartridge firearms are generally referred to by the cartridge name, they are still lumped together based on bore diameter. For example, a firearm might be described as a ".30 caliber rifle", which could be any of a wide range of cartridges using a roughly .30 inch projectile; or a ".22 rimfire", referring to any rimfire cartridge using a .22 caliber projectile.

Jerry Miculek is a speed shooter and competition shooting instructor. He is renowned as one of the fastest revolver shooters in the world, emptying a five-shot revolver in 0.57 seconds in a group the size of a playing card.^{[citation needed]} Miculek currently holds five world records in exhibition revolver shooting.^{[citation needed]} He is married to Kay Clark Miculek, herself an accomplished shooter, holding numerous national and world titles.

In addition, Miculek is an experienced gunsmith who tunes and adjusts his own revolvers for optimum function.

He is endorsed by Smith & Wesson, who created the S&W Model 625JM model in honor of him.

An avid shooter, Jerry holds numerous national and world shooting titles:^{[citation needed]}

Miculek also demonstrated the ability to fire five shots from a revolver on target with a S&W Model 64 ported barrel revolver in 0.57 seconds on September 25, 2003. This nearly matches the record held by Ed McGivern of 0.45 sec (first shot on 9/13/1932, reproduced 4 times on 12/8/1932). Originally recorded as "two-fifths of a second", the resolution of the timing equipment in 1932 was only 1/20th of a second, so the actual figure could have been anywhere between 0.40 and 0.45 seconds.^{[citation needed]}