What is the relationship between the diameter of a projectile and caliber?

Question

it's a mystory to me how a police issue revolver and a 5 inch navy deck gun can both be 38 caliber. i have studied bulistic coefficiant but have not come across anyone who can explain this to me.

Answer 1

That's because a 5" deck gun is NOT .38 caliber. You are misinformed.

Answer 2

It usually means the diameter in mm or fractions of inches.

Answer 3

They both have the same diameter, (Cal = Dia), but the difference is the force generated behind the projectile.

Also, the way the projectile is jacketed or not and the length of the barrel that it is forced from determines velocity.

The force the projectile generates is determined by its mass and velocity.

A projectile moving slower with more mass has more momentum than a faster and lighter one.

i.e. a .45 cal bullet at 200gr traveling at 800fps has more knock down energy than a 9mm or .38cal at 148gr traveling at 1050fps.

Answer 4

Diameter of projectile is actual measurement.Caliber is dia. of bore times length of barrel. See Gerald Bull on studies of artillery. He determined ideal is 45 times bore for most effective length.

Answer 5

caliber is the diameter expressed in fraction of an inch.

50-caliber = 0.50 inches diameter

44-caliber = 0.44"

38-caliber = 0.38"

5-inch deck gun would be in mm = ~125mm

Answer 6

You are mixing up two definitions for caliber.

Caliber is a diameter of the bullet or the barrel, usually in inches. The 38 caliber bullet is actually about 0.357 inches, because Colt  named his rounds after the inside diameter of the barrel, rather than the bullet diameter. At that time, tolerances were much looser than now, and left a lot of room between the bullet and the barrel.

The naval gun uses a different (often British) definition of caliber, which is the barrel length compared to the projectile diameter. Thus, a 5" tube that is 38 caliber is 5x38 or 190 inches long or 15.8 ft.

So, for a 4" barrel, a 38 caliber barrel would be 11.2 calibers (4"/0.357).