The rifled parrott gun in this mock Northwest Bastion of a pretend Fort Sanders (miles away from where the original sat) is just one of the anachronisms the reenactor community puts up with. The only big guns in the bastion in 1863 were a brass 12-pounder Napoleon of the First Rhode Island Light Artillery and a 3-inch rifle of the Thirty-Fourth New York Battery.
But, then, the walls were red clay, not wood, there were no cotton bales on the bastion’s parapet, and there was a wedge-shaped clay ramp at the bastion’s apex (between the two Parrotts in the photo) where the fort’s commander, Lt. Samuel Nicoll Benjamin, had rigged the Rhode Island 12-pounder to be run up to fire over the parapet. En barbette, as it was called.
It did so, firing two quick rounds of canister at the attackers, and then was moved to the right wall embrasure to make room for the infantry defenders. When the 3-inch gun’s horses spooked and ran away with the gun’s ammunition limber, the First Rhode Island’s Captain William W. Buckley had his gun crew remove the 3-inch and replace it with the 12-pounder for the duration of the fight.
KNOXVILLE 1863, the novel 
Yep, that is a replica 10 pounder Parrott rifle that was used at the 150th anniversary reenactment of the battle of Fort Sanders. I guess you could say it was “standing in” for Buckley’s 12 pounder Napoleon, or for the 20 pounder Parrotts of Benjamin’s 2nd US artillery, that were in the fort itself, but not in the bastion, IIRC. The other gun in the photo is a 3 inch ordnance rifle.
Writers have very often mistakenly proclaimed that the model of the northwest bastion is a full size reconstruction of the entire Fort Sanders. The reconstruction is actually only the northern half of the tiny corner section of the fort known as a “bastion”, about 10% of the size of the entire Fort Sanders. And the actual Fort Sanders was just one link of a chain, one of 16 forts, plus trenches, artillery redoubts, rifle pits, and fortifications that encircled what was then Knoxville, just the downtown section of what is Knoxville today.
Another mistake I encounter quite often is the quote that “800 Confederates died at Fort Sanders.” 800 is actually the total of all CS casualties, killed, wounded, missing, and captured. Either way it was a gruesome and lopsided battle.
The replica of the northwest bastion of Ft. Sanders was originally built as a movie set for “Its Memory Alone Remains,” a 30 minute documentary of Fort Sanders shown as part of the University of Tennessee McClung Museum’s Civil War exhibit.