JS44, I regularly shoot 2 Pietta '58 Remingtons with R&D conversion cylinders chambered in 45Colt. That load is 7.0 gr Unique and a 200 gr roundnose flatpoint. The guns are standing up to that quite well, soft as they are. The Navy Arms barrel unscrewed, but I locktited it in, it stayed there. I doubt that full loads of fffg black powder in the percussion cylinder will hurt the gun. All these Italian guns are of better steel than the originals. I shoot 30 gr of fffg with a Wonder Wad and Bore Butter in them as percussions. I did throat the barrels, however, as Pietta's throating leaves a lot to be desired. 25gr fffg in an 1860 shouldn't be a problem either, as long as it's a steel frame.