I've always loved this picture. Thanks for posting it along with the link to its caption, EarnieMac.
The two pieces in the foreground are clearly 3" Ordnance rifles but does the third appear to be a fatter bronze piece? Hard to tell and the the rest are even harder to discern. Interesting that the caption at the Wikimedia Commons link describes it as being
two Federal batteries, A 2d U.S., and C & G 3d U.S. (Consolidated), when from the photo it looks like a single typical early war six-gun Federal horse battery, with six pieces up front each followed closely by its associated caisson and all cannoneers horsed. Perhaps more guns are behind the camera? Both were recognized horse batteries and both were armed with six 3" Ordnance rifles at Antietam a short three and a half months later. (Curt Johnson & Richard Anderson,
Artillery Hell , p. 83 (citing a Memorandum in Gen. Hunt's papers).) They together with Batteries B & L (Capt. Robertson) and M (Capt. Benson), both of the 2d U.S., comprised the famous Horse Artillery Brigade of the AoP. This seems worth looking into....
A short internet search found the image in the Library of Congress with a different caption:
https://www.loc.gov/item/2014646906/ It credits the picture as showing "the C & G Batteries of the 3rd U.S. Artillery" only. That makes sense. Looking at that clearer image also causes me to think that the visible guns are all 3" Ordnance rifles.
So where did Wikimedia get their description? They say it is from
The Photographic History of the Civil War (1911). I got out the ladder and pulled down my Blue & Gray Press reprint copy. There in vol. I, p. 286 (the reprint combines each two volumes into one), I found the image and it is identified as being a picture of Gibson's Batteries C & G only.
The caption at the original Wikimedia Commons link also misidentifies Battery A's commander as Harry Benson. Battery A, 2nd U.S. is one of the most famous batteries of the war and was commanded by Capt. John C. Tidball in 1861-63. Capt. Henry Benson commanded a different horse battery, M 2d U.S., which was also a six-gun 3" Ordnance rifle battery in the same brigade. Capt. Benson was mortally wounded in early August 1862 at the second battle of Malvern Hill by a shell burst from one of his own guns. So how did Wikimedia Commons get so far off? Looking further I see another link and that leads to the image in an archive.org image of a different page in an original copy of
The Photographic History of the Civil War, page 38 (actually 33) of vol. V, Forts & Artillery. That equates to vol. III of my reprint copy and there I find it, in a portion of the book contributed by a different author, O.E. Hunt, a captain in the U.S. Army. Hunt appears wrong. Not only did he misidentify the image as also including Battery A when it clearly is of a single six-gun battery, but he also got the name of Battery A's famous captain wrong. The caption written by original contributor and Ohio University history professor Henry W. Elson identifying the picture as being only of Battery C & G appears to be the correct one, as the Library of Congress evidently also concluded. And with that a very fine evening has been entirely consumed....
Here is the LOC image: