Big Gee in Action
From left to right: .22 lr, .380 auto, .38 special, 9mm, .357 magnum, .40 S&W, .45 acp, .44 magnum, 7.62x39mm, 8mm
The picture is just a tiny fraction of bullet calibers and types in existance. Those particular ones where chosen simply because I had them on hand when it came time to take the picture. The last two are rifle bullets and the others are for handguns and carbines.
Caliber:2 a : the diameter of a bullet or other projectile b : the diameter of a bore of a gun usually expressed in hundredths or thousandths of an inch and typically written as a decimal fraction <.32 caliber>.
Source: Merriam-Webster
This measurement tells how "fat" the bullet is. Note that greater diameter does not automatically mean more power. In most of the world caliber is measured in millimeters. In the US caliber is nominally measured in inches such as .50 caliber (half inch). In reality that isn't always exactly true. For instance .38 Special and .357 Magnum are the same diameter. In practice this is only of concern to reloaders who need to know the real diameter of the case.
Length is not part of the measurement, however, length is often "understood" For instance someone talking about 9 millimeter rounds probably means 9x19mm not 9x17mm. Where there can be confusion caliber is often quoted as width x length (7.62x39 vs 7.63x54) or caliber + special designation (.45 ACP vs .45 Colt).
My guns shoot a range of ammo from weak .22 lr up to the fairly powerful 8mm mauser round. Below is a table with numbers probably not of any interest to you. Muzzle Velocity is the speed of the bullet as it leaves the barrel. Muzzle Energy is Velocity x Weight.
Each caliber bullet is further differetiated based on it's weight and construction. It is these two things plus velocity that will determine how the round behaves in the air and at the target. The table below covers the major types of ammo contruction.
| Photo | Abbreviation | Meaning | Definition |
|---|---|---|---|
 |
LRN | Lead Round Nose | Plain lead bullet. Can also be called "Ball" or "Hardball". |
 |
WC | Wad Cutter | A lead plug completely seated flush with the case. Target revolver ammo. |
 |
SWC | Semi-Wad Cutter | Plain lead bullet with the nose cut off. Target revolver ammo. |
 |
FMJ | Full Metal Jacket | Standard bullet completely encased in copper. Probably the most common. |
 |
HP | Hollow Point | Bullet with a hole drilled in the tip to help the bullet expand on impact. |
 |
SJHP | Semi-Jacketed Hollow Point | Like regular HP but with a copper sheath covering part of the bullet to control expansion on impact |
 |
JHP | Jacketed Hollow Point | Like HP but with a copper sheath covering the whole bullet to control expansion on impact |
