I have collected some videos here with examples of live firing explosive shells from Civil War cannons, using full military powder loads, and with cameras placed near the impact site. Also included is an attempt to simulate digitally what being shelled by 150 cannons on July 3, 1863 may have sounded like.
This is a more visceral experience than you will never get at a reenactment.
Live firing full military powder loads from parrot guns at the annual Historic Artillery Match, Camp Grayling Michigan National Guard Base:
Downrange shell bursts by 5th Massachusetts Battery at Fort McCoy, Wisconsin in 1990:
Special live fire target shooting against 55 gallon drums at Antietam battlefield:
A computer simulation of what Pickett's Charge artillery may have sounded like (best heard on a home theater system):