That H&R trapdoor is not an antique; they were made in the 1970's-80's. It should be more than strong enough for modern factory smokeless powder loads (or the equivalent), so I don't think black powder is needed.
The problem is that I don't know what a cleaning rod retaining pin is. The ramrod on the original rifles is held against the ramrod stop by its own spring tension, and the ramrod stop is held in by the upper band spring pin. Most of the H&R's were carbines, and the carbine has no ramrod.
Jim