I had the same question years back. My parents have an old dairy barn with an 80' long basement. It sits next to the road and well within 300' of their neighbor's house. I had their permission to target practice in the basement if I cleared it with the police first. I made a quick call to the barracks (Troop B, North Canaan) to double check that everything was kosher. The seargent on the other end of the phone said indoor firing was unregulated and I could proceed. He did emphasize the importance of good ventilation.
But that's a specific instance. Give your local PD a call for some input.
It's probably a gray area. I can't imagine it's OK to owna multi-family dwelling, occupy the 2nd floor, and think it's OK to fire in your living room, no matter what the backstop.
GunFire - are you local to CT? If so, and you seek local PD input, please let us know how you make out.
Just a guy