For any items not listed here, a good rule of thumb is to look at the context of the comment in question. Usually one can use that in combination with the below list to glean the meaning of a given acronym.
General conversation
AFAIK = As far as I know
AKA = Also known as
BTDT = Been there, done that
FWIW = For what it's worth
IIRC = If I recall correctly
IMO = In my opinion
IMHO = In my humble opinion
IMNSHO = In my not so humble opinion
IYKWIM = If you know what I mean
KISS = Keep it simple, stupid
LMAO = Laughing my [backside] off
LOL = Laughing out loud
MSM = Main-stream media
NSFW = Not safe for work
OTOH = On the other hand
ROFL = Rolling on the floor laughing
ROFLMAO = Rolling on the floor laughing my [backside] off
SOP = Standard operating procedure
WTF = What the [flip]
WTH = What the [heck]
YMMV = Your mileage may vary
Events and Gear
BOB = Bug-out Bag
HD = Home-defense
RSC = Residential Security Container (AKA a low-end safe)
SD = Self-defense
SHTF = [Stuff] hitting the fan
TEOTWAWKI = The end of the world as we know it
Gun terms
AOW = Any other weapon (an uncategorized NFA weapon)
BP = Black powder
BUG = Back-up gun
DAO = Double Action Only
EBR = Evil Black Rifle
MOA = Minute of arc/Minute of angle
SAO = Single Action Only
SA/DA = Single action/Double action
SBR = Short Barreled Rifle
SBS = Short Barreled Shotgun
SMG = Sub-machine gun
Ammo terms
ACP = Automatic Colt Pistol
BMG = Browning Machine Gun
FMJ = Full metal jacket
GAP = Glock Automatic Pistol
JHP = Jacketed hollow point
LR = Long Rifle (as in .22LR)
LRN = Lead round nose
Carrying
CCW = Concealed Carry of Weapons
CHL = Concealed Handgun License
CWP = Concealed Weapons Permit
EDC = Every Day Carry
IWB = Inside the waistband (method of carry)
OWB = Outside the waistband (method of carry)
Government
BATFE = Bureau of Alcohol, Tobacco, Firearms, and Explosives
FFL = Federal Firearms License (or one who has such a license)
SCOTUS = Supreme Court of the United States
USSC = United States Supreme Court
Sales
FTF = Face-to-face
NIB = New in box
SPF = Sold pending funds
Rights and Ownership
2A = Second Amendment
AWB = Assault Weapons Ban (usually referring to the Violent Crime Control and Law Enforcement Act of 1994)
FOPA = Firearm Owners Protection Act of 1986
GCA = Gun Control Act of 1968 (sometimes referred to as GCA68)
JPFO = Jews for the Preservation of Firearms Ownership
NFA = National Firearms Act of 1934 (or a weapon covered by the act)
NRA = National Rifle Association
NRAILA = National Rifle Association Institute for Legislative Action
RKBA = Right to Keep and Bear Arms
SAF = Second Amendment Foundation
Competition
IDPA = International Defensive Pistol Association
IPSC = International Practical Shooting Confederation
USPSA = United States Practical Shooting Association
Online
APS = Armed Polite Society
ARFCOM = AR15.com
TFL = The Firing Line
THR = The High Road
WTA = We The Armed (this forum)
-T.