It was a dime. No way a quarter would go down any 12 bore. The rule of thumb was if a dime wouldn’t enter the muzzle of a 12 gauge, it was a full choke or tighter.
In reality it wasn’t quite that simple. While generally true, choke is measured by the amount of constriction from the bore diameter to the final choke diameter. So it is possible to have a slightly undersized bore with only enough constriction to be considered a modified choke and still not have a dime enter.
