The marker for Battery M, 2nd U.S. Artillery is on the north side of the Boonsboro Pike east of Rodman Avenue. (Boonesboro Pike tour map)

From the marker:

U.S.A.

Battery M, 2nd U.S. Artillery 

Lieut. Peter C. Hains U.S.A. Commanding.

(September 17, 1862.)

Horse Battery M, 2d U.S. Artillery crossed the Antietam, by the Middle Bridge, in the forenoon of the 17th, and, preceded by the 4th Pennsylvania Cavalry, Col. Childs Commanding, and Battery A, 2d U.S. Artillery, advanced by the road to this point and went into position, one section on the right of the road, the other on the left. The battery was subjected to a heavy fire from the Confederate Artillery on Cemetery Hill, and from infantry skirmishers posted behind fences and stone walls. After losing three men killed the battery was relieved by Battery K, 5th U.S. Artillery, and withdrawn to replenish ammunition. At about 5 P.M., it returned and went into position in the right of the road, its left a few feet from this point, and directed its fire at the Confederate Infantry on the Piper Farm, which was continued until nearly dark when it recrossed the Antietam.

No. 93.

See more on the history of the 2nd United States Artillery, Battery M in the Civil War.