Jesus H. Christ. Chill out. Do whatever makes you happy, but if anything happens, verify who your target is before you shoot -- don't shoot at sounds, make damn sure it's not your parents being idiots and not acknowledging you when you're yelling at them to identify themselves, or maybe some idiot friend playing a prank on you.
Personally I think you're way overreacting, but I didn't hear the phone call, and I don't know if you're in a bad area or someplace off in the hills or whatever. More than likely, whoever called was just some friend of theirs who is stuck at a gas station calling from a payphone.
If your parents have a cellphone, call them. The proper response to this isn't to play cowboy or pretend to be in a movie.
ETA: I didn't notice the first time through that you'd been getting these calls repeatedly. Did you tell them before? Do you have caller-ID? I would suggest handgun training; an AR-15 is unwieldy inside a house, and the bullet will go through a lot more walls than a .45 would. A shotgun is a better choice.