One can clearly see the original "NOT FOR BALL" stamp but I'd think that was because 8 Bore fouling guns generally had tight chokes - since the barrel has been cut, that shouldn't be an issue. I will, however, insure that the barrel has no choke anyway.

1200BAR is a pretty high proof pressure (+/- 17500psi)
Ive read of an 8 being measured at 11,000psi using 10 Drams (270grs) powder with an 880gr .850" ball in a 24" rifle 1500fps. I'd be happy breaking 1000fps with a manageable charge.

Funny thing, the "NOT FOR BALL" stamp has been struck off to read more like "NOT FOR ALL"
I think that's appropriate.