LOL....my Gawd. How many people does it take to translate "tbb" for this poor guy?? Anyways, the correct term for a tight bore barrel is "TN" - for tight/narrow, which it has been for the last 3 decades. If we start assigning our own terms and definitions to various items that already have them, everyone would need a translator to interpret the different acronyms from player to player.
So if you don't know the correct acronyms, please type out the all the words rather than creating a whole world of confusion, especially for the new players.