Thank you all for the replies.

I am currently waiting for a call regarding any markings on the breech of the blank which hopefully will shed some light on the composition. It is sitting in the back of a friend's Subaru.

I was going to make a drop in for a pre existing rifle hence the 15/16. Worst case I build another rifle and cut a wedding band to drop some weight.
The only rifle that I own that could handle the blank as is is an underhammer. That would make a nice hunting rig in itself and would be a bit less work than fitting to a hammer gun.
Thanks

Tom