If you are interested in historical accuracy, good rule of thumb is iron trimmed rifles are seen post 1800. Makers like the Beans, Bulls, Gillespies and of course the Hawkens were known for their iron mounted guns. Some used a mixture of brass and iron at times. All of these makers mentioned above are southern or western of origin and were active from 1810-1870. I may be wrong but some mid century 1830-1860 Pennsylvania made Henry and Leman trade rifles can be found with iron furniture.
An 18th century longrifle is almost always trimmed in brass. Very very few examples of iron trimmed 18th century rifles exist and the ones that do are debated.
An iron trimmed early Lancaster makes a striking rifle, but if historical accuracy is your main concern it should be trimmed in brass.
