These cannons are much smaller than the other cannons used here in 1863 such as the Parrotts or Napoleons. These weapons are quickly identifiable as the tubes are black, very smooth, and almost always in terrific shape and represented at the time, some pretty serious weapon technology. There trunnions are devoid of markings. Their tubes do not have the breech band seen in the similar looking Parrotts. The 3-inch carriages are also always equally pristine. This is an 1861 model but was manufactured in 1863 and may have seen service at Gettysburg. This model was also mass-produced by the Confederates as well, but not nearly as well made as their Union counterparts. The stamp on the outer muzzle was comparatively well-preserved as I was able to read the stamp muzzle and trace back the manufacturing origins but the gun to the right or front of the monument was battered and quite worn and took some doing and minor investigating to trace its origins.
Calvin Gilbert owned a Foundry which bore his name in Gettysburg. This man was responsible for creating all of the cast iron battery tablets (including this one) at this park. His work did not end there as his foundry created many of the carriages for the historic cannons at Gettysburg. Although almost all of the cannons are from the Civil War, their carriages, sights, and caissons have been restored at his foundry in the early part of the 20th century. GIlbert was in his 70s when he did this work and competed work at other battlefields as well.
The two 3-inch guns and the Hampton's Battery C and F Pennsylvania Artillery Monument are located on Wheatfield Road, 190 feet east of the intersection of Wheatfield Road and Emmitsburg Road (Business U.S. 15), and on the right or south side of Wheatfield road if traveling east. The monument is unusual in that when facing it from the road, you are not looking at the front of the monument but rather the left side. This is because the monument faces west and the road also runs east/west. The rifles point in a southwest direction. This area is near the tenth stop for the independent auto tour as well as the guided tour. I saw a number of cars unload just up the road with a park ranger always at the wheel, then emerging with visitors and then leading the way on foot and explaining the doings here at the Peach Orchard during the battle. 
Parking is difficult but available at enlarged shoulder sections intermittently spaced throughout this area. Stay off the grass less you be ticketed. I visited this monument on Thursday, July 5, 2012 at 11:48 AM, at a 619 foot ASL elevation. I used my Canon PowerShot 14.1 Megapixel, SX210 IS digital camera.
My SOURCE for all things weapons at Gettysburg provided me additional information about this weapon which did not appear on the muzzle. I used red to designate that data. The rest is as it appears exactly on the muzzle, starting at the top or 12 o'clock position and moving clockwise.
No. 606 ......PICo ......1863 ......T.T.S.L. ......816 ......FDY #654 ......GRVS 7RH
No. 606 refers to the Army registration number, a way for the military to keep track of the guns. PICo refers to the Phoenix Iron Company in Phoenixville, PA, the foundry where the fun was manufactured. I believe when the Rebels were able to capture one of these guns, they copied it and produced their own, although this one was federally made. 1863 refers to the date of manufacture even though this is an 1861 model. 816 lbs refers to the weight of the firing tube. This number is on the bottom of the muzzle. Each gun is usually very unique and has its own weight which distinguishes it from every other gun, like a fingerprint. I am not sure how they managed to use mid-19th century forging technology to create identical weighing cannons/rifles. T.T.S.L. are the initials of the inspector who gave the gun a once over before it was shipped out to the Federal army. The initials stand for the very famous Union Army Inspector, Major Theodore Thadeaus Sobibski Laidley. FDY #654 is information not found on the muzzle but provided by the website. This is an internal control number specific to the foundry. The carriages were approximately 900 pounds which makes this entire weapon over 1,700 pounds. The GRVS 7RH, more information provided by my source, refers to number of rifling grooves, left or right twist. In this case, 7 right hand twists were made to rifle this cannon.
About the Foundry
The Phoenix Iron Works (1855: Phoenix Iron Company; 1949: Phoenix Iron & Steel Company; 1955: Phoenix Steel Corporation), located in Phoenixville, Pennsylvania, was a significant manufacturer of iron and related products during the 19th century and early 20th century. Phoenix Iron Company was a major producer of cannons for the Union Army during the American Civil War. The company also produced the Phoenix column, a significant advance in construction material. Phoenix Iron Works is a core component of the Phoenixville Historic District, a National Register of Historic Places site and in 2006 was recognized as a Historic Landmark by ASM International. SOURCE & SOURCE
About the Inspector
Theodore Thaddeus Sobieski Laidley (1822 - 1886) was an army officer specializing in ordnance. He was also an inventor and author. Laidley was born in Guyandotte, Virginia April 14, 1822. He graduated in 1842 from the U.S. Military Academy and chose a commission in the ordnance corps and served in the Mexican War. At the end of the war, he returned to the arsenal at Watervliet, New York, as Assistant Ordnance officer. Laidley was put on detached service to write a new ordnance manual published in 1862 which served as an important guide during the Civil War. He served on several ordnance boards designing and testing weapons, taking out eight patents. He was later president of the commission to test the strength and value of all kinds of iron, steel, and other metals at the Watertown, Mass. arsenal 1871-1881, retiring in 1882. Laidley wrote government reports and A Course of Instruction in Rifle Firing, Philadelphia, 1879. Theodore Laidley died in Palotka, Florida April 4, 1886. He was inducted into the Ordnance Hall of Fame in 2008. SOURCE, SOURCE, & SOURCE
About the Gun
The second most common rifled field artillery in both Armies generally, and the most common on the Maryland Campaign, the 3 inch Ordnance gun was made of hammer-welded, formed, machined iron. It was popular because of its accuracy and reliability, at least those examples built in Federal shops. Less precise machining and lower-grade iron gave their Confederate counterparts more trouble. Those built by the firm of Burton and Archer were know to be problematic. The 3-inch rifle normally fired Hotchkiss or Schenkel shells that weighed between 8 and 9 pounds. In an emergency it could use 10-pounder Parrot ammunition. It could also be used to fire cannister but, as a rifle, was not as effective with this as howitzers or Napoleons. The maximum range of this weapon was 1830 yards, with the barrel/tube Length 69 inches, the bore 3 inches (hence the name) and the total combined weight (already mentioned above), approximately 1720 pounds. Johnson, Curt & Anderson, Richard C., Artillery Hell: Employment of Artillery at Antietam, College Station, TX: Texas A&M University Press, 1995; and
Schwartz, Peter, Artillery at Gettysburg online. SOURCE & SOURCE.
The tablet, which marks the position of Hampton's Battery on July 2-3, 1863 reads as follows:
(Front):F. Penna. Light Artillery
Organized at Pittsburgh
Hampton's Battery
(Right): From June 3 1863 to March 25 1864 Batteries F and C served as a consolidated battery.
July 2, occupied this position from about 5 to 6 o'clock p.m. July 3, with the left centre on cemetery ridge on left of First Volunteer Brigade Reserve Artillery marked by tablet. 24 men from battery F were detailed to Battery H 1st Ohio Artillery posted in the Cemetery during the battle.
(Left):Mustered in Oct. 8, 1861
re-enlisted Feb. 27, 1864
Mustered out June 26, 1865
This area is loaded with all types of cannons and guns, a veritable outside museum. I would suggest allowing up to an hour to fully inspect all the tablets, monuments and weapons in this area.