All of the "2 1/2" shells I have seen have in fact been 67.5 mm.
I use the Eley grand prix and impax in paper.
I have never seen any specs but I believe these are fairly mild loads.
I have used black powder 70 mm reloads in my Damascus guns without issue having read both Thomas , Burrard and discussions here before.
However I now use the paper hulls from the aforementioned Eleys,which last longer as well.
Anyone have any idea what pressures these factory Eleys produce?

GDU