How much muzzle constriction there is.
If you mean a choke bored barrel, it designates the amount of constriction a shotgun barrel has.
Modified Choke
A barrel with a full choke.
They probably denote what choke the barrel has.
the barrel is narrowed at the muzel for a tighter shot pattern.
This means there is no "choke" or narrowing of the barrel at the muzzle.
At the end of the barrel
How much the shot is constricted as it leaves the barrel.
They are a code used to denote choke size.
choke of sears model 20 12 gage plain barrel shotgun has 3 stars on barrel
They came in various barrel lengths and choke combinations. If it is not marked on the side of the barrel, you will have to pattern the shotgun to determine the choke
A shotgun barrel with a cylinder bore choke.