Major Waldrons Sportsmans assoc. in Dover has a indoor range, it is a private club and costs arond $160 or so a year. You can shoot almost any time if you go through their "keyholder qualification "
From their website
Indoor Range:
Year round shooting
Ten movable shooting stations
Shooting distance up to 25 yards
Indoor Range Open Shooting: Open shooting is only available on the first Thursday of the month for the Summer. It will be open every Thursday starting September 25th. The range is open to members and non-members alike. Standard range fees apply - $3 for members, $10 for non-members. Note: If no one shows up by 7:15 p.m. the range officer will likely lock up and leave.
Note: All guests are required complete a "Release and Assumption of Risk" form prior to using the facilities.