the barrel should be iron or iron welded if its that old
Not necessarily - cast aka crucible steel barrels, which were drilled from solid bar stock, started becoming popular in the 1830's and were widely used by the 1860's. By the 1860's the Bessemer process for making steel made it much more widely available and generally replaced WI, but although WI continued to be made and used up through the 1930's, it was used less for firearms, and was more often used in construction projects in the latter days due to it's rust resistance.
But on the other had the detergents are going to go where the oil does so what will they do to Iron or early types of steel .
FWIW - The atomic structure of iron and simple steel (iron and steel are not made up of molecules but are rather a crystal lattice structure), whether made today, 140 years ago, or 1,500 years ago are not all that much different.
The base of both iron and steel is ferrite - generally the only real difference between wrought iron as used for barrels up until the mid-19th century and the simple steels of then (or today - todays 10XX series steel makes a good substitute for the steels of the past since it is a simple steel like made then) is the carbon content and the fact that WI usually has a fairly high silicate content due to the manufacturing methods of the day.
Despite "common" knowledge WI can and often does have carbon at levels similar to mild steel (.1 to .4%). The thing that really separates the two is the "stringy" structure of WI due to the silcate inclusions. One source of the silcate inclusions was sand which was often used as a flux during the processing of WI.
Refine WI by heating and beating or by burning off the impurities (such as when making puddle iron or by forging) and it's not much different structurally than mild steel of similar carbon content.
Even todays more exotic alloyed steels generally have no more than 5-10% alloying agents added to the basic ferrite.
Actually pure iron, ferrite, is in fact quite rare outside of labs - even the so-called plain steels and irons of the past often had naturally occurring alloying agents such as manganese which aids in hardening or copper. Other "agents" could/would be added during processing such as carbon and sulphur (from the coal). Sulphur is not a good thing and was one of the "problems" with old steels and irons.
I'd just use the brass brush and hold off on the 4/0 steel wool until you see what you have. Another less abrasive option is the nylon scrub pads.
Regarding driving out the pin - pins can be tapered so if it seems tight coming out in one direction try the other before getting too aggressive.