My double was made of "fluid Steel" - back around the turn of the 19th century. No problems in it. Any gun that is really rough, ie: pitted badly or deeply, might slough off plastic from abrasion, but then, it will slough off everything else as well. Deeply pitted barrels or barrels with wide pits should be shown a great deal of caution, especially when damascus - the holes can tend to go through almost to the surface.
: I would use only smooth bores. I can fore-see no reason for problems with relatively smooth bores of any age. I know the boys used to shoot plastic wads in their black powder shotguns, until they had to get the plastic out. From then on, it was a couple hard over-powder card wads down first - then plastic.
; One old .74 cal. single barrel gun I patterned put out almost improved modified patterns with 2-1/8"cards, pink steall shot plastic wad, 1-1/8oz. #6's, thin "BB" wad loaded with 3 drams 2F. The shot loads were used for the 'trap' event on a trail-walk at Hefley Creek Rondy. All the rest of the shots on the trail were with round ball - I won 2nd or 3rd place on that trail walk. That Gun, an H. Wahl, was made in Birmingham, I think, around 1860. The plastic didn't hurt the bore at all, nor even foul the pits. I had polished the bore as well as possible with a tight cloth on a mandrel, in the electric drill- turned full speed, and with chrome polish on the cloth. It helped smooth the entrance and exits to the pits, I assume.