This was an entirely new gun pattern Tredegar developed in early 1861 for the popular 6 pounder. It is made of iron. Previously, it was accepted practice for all field guns to be brass or bronze. Iron was lighter, stronger and less expensive. One of the reasons Robert Parrott's new 2.9 inch guns were initially rejected is because they were iron. The primary field guns brought by the Union onto the Bull Run battlefield were bronze 6 pounders rifled to fire a 3.8 inch James shell. Yes, they had some Parrotts but those guns had not been formally accepted by the U.S. Ordnance Dept. yet. So, this Tredegar gun is a Rifled 6 pounder, not a 3 inch Rifle. By 1862, they had a different 3 inch Rifle pattern. This early 1861 pattern is based upon the Model 1841 smoothbore 6 pounder. Instead of boring for a 3.67 inch cannonball, it had a 3 inch opening. Just weeks earlier, Tredegar had cast and sent 4 bronze Rifled 6 pounders to Mississippi that look exactly like the ornate old 1841 pattern guns except they were bored to 3 inches. Two survive and are on display in Mississippi. (see an example on Bull Runnings website.)
1) The first item below is taken from the Tredegar gun book. It reports the production of gun #1151 made of 1,500 pounds of "Gunther's" iron. Again, it is called a "6 pounder Rifle."
2) The second page below from Tredegar's production records reports the shipment of this "6 pdr. Rifled Gun" (i.e., 3 inch) to the state of North Carolina along with various implements. Note they also made 8 inch and 10 inch guns and ammo for the state's coastal fortifications like Fort Macon.
3) Days later, Tredegar sent six boxes of 6 pdr. Rifled shot (i.e., 3 inch Archer shot) to NC along with additional ammo and guns. Remember, Tredgar made no 3 inch Archer shells prior to July 28th, 1861.
Woodenhead