It refers to the size of the shot pellet. It is the largest size of "birdshot", and each pellet is about .190" (4.83mm) in diameter. There are about 62 of these pellets to one ounce.
No
none
yes
.410
gun powder
Any gun store sells it. Commonly shotgun shells #8 or #9 shot.
Shotguns fire shot shells. Handguns can fire specially made shotshells.
Many fine 20 gauge shells are made.
BB is one of a number of standard pellet sizes of bird shot there are also "B" and "BBB" sizes, but BB is the most common. (Ball Bearing)
Have it checked out by a gunsmith before you try it.
depends...mine holds three when shooting 2 3/4" shells
Could be any of them as the "00" does not denote caliber, but the size of the shot contained in the shell.