The Gun included the AIP sets is pretty good. The scale of the figures might be too big or the gun is a bit smallish. You decide. I'm comparing it with the Heavy Artillery carriage and both types of barrels included in the AIP sets.
Here's a period photo. (Not a Parrott barrel)
Here is a gun at Fort Pulaski in Savannah, Georgia.
Here is the AIP gun shimmed to match the base height of the figures.
Another shot of the Fort Pulaski 30 lb Parrott.
The AIP gun.
It might be that the angle is off and the wheels are not shimmed in the photo above.It's a bit off from the figures but a great little model for the price. Not many siege guns are available in model or toy sets. The gunners and barrels included in the sets have other uses if one could scratch build fortress carriage guns.