Answer: When it is a bronze rifle rebored to masquerade as a 12-pounder.
I visited Chancellorsville earlier this week. At Hazel Grove they have redone the confederate battery since the last time I was there (Been several years) and I've learned a lot since then. I noticed that one of the three Napoleons didn't quite look right. Upon closer examination I noticed the breech was about three inches smaller in diameter. I looked at the muzzel. Here's what I saw:
You can just see the rifled original bore several inches inside the tube. Unfortunately all the maufacturing information was ground off the muzzel. I would really have liked to know what this gun began life as. Does anyone in the peanut gallery have a hypothesis?
The Park ranger told me they also have a 3-in ordnance rifle maquerading as a parrott somewhere in the Fredricksburg/Spotsylvania Military Park as well.
What you have there is a "false Napoleon". These are found mostly at Gettysburg and were 6-pounder guns and rifles that were modified by the Park Commission around 1895. The commission wished to arrange the park to mark the battlefield properly, with careful location of monuments to different troops. Meticulous effort was given to placing in each battery position the same type of gun as was present 1 to 3 July 1863. With a limited supply of artillery pieces, at some spots two or four cannon were used to mark the location of a 6-gun battery. Enen so, there were not enough Napoleons, Ordnance rifles or Parrotts to go around, but Army depots contained an ample supply of 6-pounder guns and rifles. To remedy such imbalance, the commission engaged a local foundry and machine shop that made cast iron replicas of 3-inch Ordnance rifles and 2.9-inch Parrott rifles. For bronze pieces they simply altered 6-pounder gun tubes to represent Federal Napoleons. Thier rough machining and differences of size, shape, and stampings continue to pass unnoticed by all but the critical observer. Check out Chapter 10 of "Field Artillery Weapons of the Civil War" by Hazlett, Olmstead and Parks for more info on these pieces.
When other military parks were established Gettysburg was called upon to send cannon for display at the new sites. Twenty of the false Napoleons stayed at Gettysburg, four were sent to Antietam, three to Fredricksburg, and one each to Chancelorsville and Mannassas.
I'd seen that designation on a couple of other citings on an online directory of artillery but I wasn't quite sure what it meant. Your background information was much more useful than what I'd previously seen. Actually seeing one gives it a whole new dimension. Next time I pay a trip to Manassas I'll have to look more carefully at the Napoleons there. If I remember correctly from the last time I was there (been quite a while) I had noticed some of the bronze guns were rifled and some not. Of course, at 1st Manassas, there would have been a number of actual 6-pounders employed since things were still just starting up.