A few years back, I callled the local police and asked about firing my cannon on the 4th of July, making sure I explained that it was a muzzleloader : stuff-it-down-the-front-Davy-Crockett-style. I was told that I needed a pyrotechnics permit just to fire it in PA. (I understand that the former Mayor of Lancaster, PA, Charlie Smithgal, a Civil War re-enactor, owns 30 or 40 cannons which he fires at his place in the country.)
I then called the state police and got the same answer. I actually looked in the PA statutes and Code, and couldn't find anything, one way or the other.
There can be several ways to get you, a) in some areas barrels of a certain bore size are considered an explosive device. b) discharging a firearm in municipal limits, c) violation of the noise ordinance, if you have one, d) disturbing the peace, even e) causing a public nuisance,