The British Gun at Sullivan’s Island
The other day, over at To the Sound of the Guns, Craig Swain took a look at a very unusual piece of Confederate artillery, photographed at Fort Marshall on Sullivan’s Island, near Charleston. Looking closely at the image (available online at the Library of Congress), Craig noticed that cast into the upper side of the reinforce was a distinctive marking, almost certainly a British royal cipher. The gun in the Civil War era photo is very likely one of a handful of old British 12-pounders that, in the latter part of 1863, the Confederates had rifled and fitted with breech banding to further expand there defenses around Charleston. Those guns are marked with what looks like the monogram of George III (right, 1760-1820), which would presumably make them of Revolutionary War vintage.
There are a couple of other interesting things about this image. The first is that the gun is mounted on a field carriage, rather than a traditional seacoast mounting like the gun at left. Perhaps this was done because, as a relatively lightweight piece, the rifled 12-pounder was intended to be moved about to different positions as needed, which would be greatly facilitated by putting the gun on a field carriage.
The other thing that’s interesting is the clear view of the way the batteries themselves were covered with sod, to help maintain their shape. We’ve seen reference to this practice at Galveston, as well, and it would be essential, given that otherwise the loose sand would blow away as fast as it was piled up. The famous Confederate artist Conrad Wise Chapman completed a painting of the interior of Fort Marshall, that shows the green-sodded earthworks to good advantage:
And if you look closely, at left Chapman depicted two guns very similar to the one in the photograph, along with a tiny, house-shaped structure similar to the ready-ammunition storage seen in the 1860s photo:
Finally, since the image of the old ye olde British gun is part of a stereo pair, I would be remiss in not offering them in all their 3D glory, for for red/cyan viewing and as a wobbler:
_________
Dead Confederates, A Civil War Era Blog 
Andy, I’d forgotten about Chapman’s painting. Thanks for bringing that up!
I’m still reluctant about the positive ID on the Royal Monogram. There are surviving Revolutionary War pieces which have King George II’s monogram in a similar style seen on the Charleston gun. Likewise guns from the later decades of George III’s reign have a different monogram (with a belt forming a circle around the GR below the crown). Starting around the mid-1780s, the Master-General of Ordnance’s monogram was often placed on the chase. That second monogram is not present on the Charleston gun. From that, I’d deduce the gun was produced prior to 1785.
The carriage in question is more precisely a “siege carriage”. Although it lacks the built up cheeks of heavier siege carriages (for say the 30-pdr Parrotts), the carriage does not have the “lunette” of the field carriages (the loop where the carriage hooked onto the limber). There are the long bolts used by the crew to position the trail using poles as that leaning against the trail. And there’s the cradle and traveling trunnions where the gun was positioned during movement to center the weight. Regardless of the exact nomenclature, you are correct that the piece was so mounted as to afford mobility. These 12-pdrs were posted at several points along the Charleston line at different times, depending upon the perceived threats.
I think Chapman’s painting is from the far left end of the fortification, with the larger guns in the traverses in the background. On the other hand, the photo seems to be from the right end of the fortification, with the 12-pdr covering the beach and the Brooke looking out over the surf. If that is the case, the photographer’s camera would have been to the right (off the canvas) of Chapman’s painting, behind the last hut.