From the stone in front of the gun:

MODEL 1861
Field gun made in 1863 for Union forces
by Phoenix Iron Co., Phoenixville, PA.
Wrought iron
Diameter of bore 3 inches
Overall length 73.3 inches
Weight 816 pounds
Fired 9-pound shell 3972 yards
(2.25 miles)

Was this weapon used at Fort Monroe? The stone is suspiciously silent on this matter.