Interesting History on these guns. The Early ones were born out of the need for more firepower,larger charges, heaver projectiles. Some existing tubes were simply banded around the breech ,thus the look ,and they were guns,not rifles.
The true Parrot rifle didn't appear until Early 1862 .Still being of a gray iron ,cast ,and was still called an 1861.
They were the heavy guns for the Army and were also used by the Navy.
The 1861 Ord. Rifle was used more widely at the end of the war ,even being a three inch , advanced production of it took it from iron ,to cast steel.
Making it lighter ,and still rivaling the power of the Parrot.
Both having a three inch bore ,but the OR being 20% lighter.
The 20lb Parrot had a 3.5 in bore ,and was used mostly as an emplacement gun.
Larger Parrots were used by the Navy ,The Monitor had a pair made especially for that ship, with a barrel 12in longer than the standard gun ,with a cast steel chamber band.
The CSS Virginia used 18 ,1841 6 bounders ,neither ships guns were all that effective on their enemy's iron clad armor.
But the men inside said it was like living in Hell.
The Confederacy ,did all it could to try and take a Parrot in tact ,and even made a copy of it of their own.
Both guns were soon to be shadowed by a gun that could act like a Mortar ,and fire like a cannon ,and was half the weight.
The large bore Howitzer , This gun became the favorite of Gen ,Sherman.
It was the first time a Machined steel sleeve ,centered into a cast steel casting ,was tried ,it was successful.
Though lacking range ,it could deliver more explosive than the other two guns and was used on Atlanta.
It also could deliver Grape shot to a larger area ,without the need for a canister for containment.
Making the suicide charges of the past just that ,past.
I didn't mean this to be so long ,It is a short except from a book i have,I thought it was some good reading and i wanted to share it.