That's your chamber size. Measure your constriction (bore diameter-muzzle diameter) for a general idea, or look for another marking (often under the stock). It may be a decimal number, a series of zeros, or a choke marking. A constriction in the teens would be imp cyl, in the 20s would be mod, 30s is full, 40 and up and you're into extra full or turkey chokes.
However, this isn't always true - the best way is to measure the constriction. A bigger bore and a tighter muzzle would perform differently that a relatively tighter bore with the same muzzle.
EDIT: just saw your last update. If you can put a dime in there, it's most likely not a full choke.