Mainly for newcomers to firearms, here are some of the more common abbreviations used when referring to ammunition:
LRN - Lead Round Nose
SWC - Semi-Wadcutter
LSWC - Lead Semi-Wadcutter
FMJ - Full Metal Jacket
TMJ - Total Metal Jacket
JHP - Jacketed Hollow Point
JSP - Jacketed Soft Point
gr - Grain (refers to bullet weight, as in 115 gr 9mm, 230 gr .45 ACP, etc.)
ACP - Automatic Colt Pistol (.45 ACP)
GAP - Glock Automatic Pistol (.45 GAP)
LR - Long Rifle (.22 LR)
AE - Action Express (.50 AE...if this is your first pistol, you've got issues
LC - Long Colt (.45 LC)
S&W - Smith & Wesson (.40 S&W)
Rem - Remington (.223 Rem)
Win - Winchester (.308 Win)
WMR - Winchester Magnum Rimfire (.22 WMR)
Spl - Special (.38 Spl)
Mag - Magnum (.357 Mag)
NATO - North Atlantic Treaty Organization (5.56mm NATO)
+P and +P+ indicate rounds loaded to pressure higher than 'standard'. A 124 gr +P cartridge will have higher velocity and energy than a 'standard' 124 gr cartridge by the same manufacturer. Likewise, a 124 gr +P+ cartridge will have even higher velocity and energy. Do NOT fire a +P or +P+ cartridge in your gun unless the manufacturer explcitly states the gun can handle the higher pressures.
In general, a gun chambered for .357 Mag can fire slower (and cheaper!) .38 Spl ammunition. It's common for owners of guns chambered for .357 Mag to carry and/or practice with .38 Spl in standard or +P loads, due to ammo cost, and reduction in recoil. However, a .357 Mag round should NOT be loaded into a revolver chambered for .38 Spl. Even in the unlikely event you manage to cram a .357 Mag cartridge into a .38 Spl cylinder, if you somehow manage to fire the round, the significantly higher pressure of the magnum round will likely blow the gun up.
.223 Rem can be safely fired in a rifle chambered for 5.56mm NATO, but accuracy may suffer, due to slightly different cartridge dimensions. However, the opposite is NOT true...the 5.56mm NATO round is generally loaded to higher pressures, in addition to slightly different cartridge dimensions. While it may be possible to get a .223 Rem chambered rifle to load a 5.56mm NATO cartridge, the higher pressure of the 5.56mm cartridge will at best lead to excessive wear on the firearm, and could very likely cause a catastrophic failure of the weapon and lead to injury of the shooter.
.308 Win and 7.62mm NATO are similar cartridges, and I know of no pressure-related cautions when loading one cartridge in a rifle chambered for the other.
You will often see ammunition with a bullet size, followed by a cartridge lenth, such as: 7.62 X 39mm (or just 7.62 x 39).
Though the AK-47 7.62 X 39mm round may seem similar to the 7.62mm NATO round, they are entirely different rounds, and not interchangable at all. The NATO round is 7.62 X 51mm...a significantly longer rifle round, versus the intermediate lengh of the AK-47 cartridge.
Likewise, 9mm Luger/NATO/Parabellum (9 X 19mm) is not compatible with 9mm Makarov (9 X 18mm).
Okay, I probably should actually get back to WORK...