There was reportedly an H&H double rifle ffffed in Melbourne this year (or last) using the wrong sort of projectiles ... I wasn't there to see it, but was told about it. I think it was quite a new one, and therefore very expensive.

Some of the Melbourne members on here would know all the details, if they are willing to share the info.

Since this thread (?) maybe since its beginning, I have spoken to the gunmakers at Verney-Carron and Heym, and they didn't warn against using mono metal projectiles. SO presumably their modern firearms are OK with them.

BUT personally I would ONLY use monos with raised driving bands such as the hydros, not examples with 'driving' grooves instead. Just MO. Not going to use either most probably and stick to what I already use.