• DA: Double Action
• Pulling the trigger cocks the hammer back until pulled to a certain point. Then the hammer is released to strike the firing pin.
• Involves a heavier trigger pull (can be 8 pounds or more) and can be less accurate for some people
• Used on many handguns, especially defensive ones, both revolvers and automatics
• SA: Single Action
• The hammer is manually cocked back by the user's thumb or by pulling the slide back on an automatic. Autos with an internal hammer have to use the slide to cock the hammer (the user racks the slide manually to cock the hammer if the gun was dry fired or misfired; under normal use the hammer is automatically cocked with each shot by rearward motion of the slide)
• Involves a much lighter trigger pull (say 1 to 5 pounds). Usually more accurate.
• Used on both automatics and revolvers.

• DAO: Double Action Only
• The gun cannot be fired in SA.
• SAO: Single Action Only
• The gun cannot be fired in DA.
• DA/SA: Double Action/Single Action
• The gun can be used in both modes.
• Used for revolvers and automatics.
• For practical applications with an automatic the first shot would be DA. The rearward motion of the slide would then cock the hammer on subsequent shots making the pistol SA.
• For revolvers each shot would be double action unless the user manually cocks the hammer.
• Striker-Fired: There is no rotating hammer to strike the firing pin as in many pistol designs
• Instead the firing pin is attached in some way to a spring. The spring is compressed by some mechanism. This can be by the rearward motion of the slide, pulling the trigger back a certain distance or a more unique method as in the H&K P7.
• Pulling the trigger releases the spring allowing it to push the firing pin straight forward into the primer of the cartridge.
• Glock pistols are probably the best known striker-fired pistols though there are others. In fact, I own a Colt Model N 1908 Vest Pocket pistol that uses this type of mechanism. These were produced from about 1908 to 1946. That is not to say the Colt and Glock mechanisms are identical, just similar.
• Browning Lock-Breech Tilting Barrel: Common among a vast majority of automatic pistols made in the 20th and 21st centuries.
• Typically used in pistols using 9x19mm round or more powerful (.40S&W, .45ACP, many others)
• When the pistol is fired the slide and barrel recoil toward the shooter locked as one unit for a short distance
• After the slug has had time to exit the barrel and dangerous gases have exited the muzzle end of the barrel:
• The barrel disconnects from the slide and its breech end tilts down into the frame towards the next round at the top of the magazine
• It's guided by a slitted lug on the underside of barrel and a mating piece cut into the frame
• 1911-style pistols have a link attached to the barrel. The slide stop/take down lever mates with link attaching it to the frame when the pistol is assembled. This provides the pivot point.
• The slide continues back towards the user until it reaches the end of its travel
• The recoil spring (compressed by the previous actions) then forces the slide forward
• The breech face of the slide strips the next round off the magazine and forces it into the breech end of the barrel
• The barrel travels upward to reunite with the slide
• Less powerful rounds such as the .380ACP, .22LR, .25ACP, etc can be and usually are fired from a non-locked breech design
• The barrel remains fixed to the frame and does not travel rearward with the slide
• The Beretta 92/96 series pistols are proably the best known autos that do not use this Browning-designed system for their 9mm/.40S&W pistols
• The barrel does travel rearward with the slide for a short distance then disconnect
• However the barrel does not tilt down into the frame
• A locking block attached to the underside of the barrel disconnects from barrel (hinges on pin) and tilts into the frame
   

Cartridge Names:

• ACP: Automatic Colt Pistol
• LR: Long Rifle
• NATO: North Atlantic Treaty Organization; usually used as nomenclature for military ammunition
• WMR: Winchester Magnum Rimfire
• Parabellum:  The word Parabellum is a noun coined by German arms maker Deutsche Waffen und Munitionsfabriken and is derived from the Latin saying si vis pacem, para bellum, meaning If you wish for peace, prepare for war. The term has been used in the naming of a number of cartridges.^[1] It is also the name of an online FPS-style game.
• This information came from http://en.wikipedia.org/wiki/Parabellum and was brought to my attention via email by Dan Timbrel.
• Example: .380ACP
• .380 is the diameter of the cartridge slug (peice actually fired for gun) in inches. In this case .380 inches (380 thousandths of an inch. The cartridge name does no always accurately define the slug diameter. For instance, I believe the .38 S&W Spl and .357 magnum both have slug diameters of approximately .357 inches.
• Example: 9mm Parabellum (Luger, NATO, 9x19mm)
• 9mm is the diameter of the slug
• 19mm is the overall length of the casing

Other terms:

• Breech: The breech end of the barrel is the end into which the cartridge fits.
• Cartridge: Complete unit of ammunition consisting of a casing, primer, powder and slug.
• Casing: Containing unit of the cartridge.
• The powder is contained in the hollow cylindrical casing and a slug is fitted into the top
• A primer is fitted into a hole in the end of the casing opposite the slug or made into the rim of a rimfire round
•  
  Usually made of brass
• This is why after shooting you should "round up your brass".
• Centerfire Round: The primer is in the center of the bottom of the casing
• Once a centerfire cartridge is used it can be reloaded with fresh powder, slug and primer if the proper tools are available
• Usually very reliable
• Decocker: Control on handgun used to safely take it out of SA mode and put it in DA mode
• The hammer is usually physically blocked from contact with the firing pin by an internal mechanism while using a decocker
• The decocker function may be built into a manual safety on the gun or a separate lever
• Not all handguns have a decock function
• FTE: Failure to Eject
• Malfunction in semi-automatic weapons
• For some reason after the gun is fired the spent casing does not fully exit the action of the gun
• Usually results in a "stovepipe" (see below)
• FTF: Failure to Feed
• Malfunction in semi-automatic weapons
• For some reason the next cartridge in the magazine does not properly seat in the breech end of the barrel
• The action (slide) cannot move completely forward and lock with the barrel
• This should render the firearm useless until the malfunction is cleared; the trigger should not work
•  
  FMJ Round: Full Metal Jacket
• The slug is fully encased in a copper jacket except the unexposed end inside the casing
• Good for penetration but not expansion of the fired slug
• Grain: Unit of measure for weight
• Used to specify weights of both the powder charge used in the cartidge and the slug
• The grains listed on a box of ammunition usually refer to the slug weight
• Muzzle: The end of the barrel from which the slug exits when the gun is discharged.
• HP Round: Hollow Point
• A cavity has been hollowed out of the front of the slug
• Usually helps the fired slug expand in soft tissue or fluid at the expense of penetration
• Rimfire Round: Looking at the end of the casing opposite the slug no primer is visible
• The primer is built into the rim of the cartridge
• Cheaper to produce than centerfire ammunition but not as reliable
• Once any part of the rim is struck by the firing pin the primer is spent
• These rounds include the popular .22LR, Long and Short though there are others
•  
  Slug: Part of the cartridge actually fired from discharged weapon; projectile.
• Usually made of metallic lead and can be coated in a copper jacket.
• Stovepipe: Malfunction in semi-automatic weapons
• After the weapon is discharged the spent casing is not fully ejected from the firearm
• The action (slide) slams forward on the spent casing pinching it between the breech face of the slide and the breech of the barrel
• The spent casing looks like the venting pipe coming off a wood/coal burning stove, hence the name