Here are some photos of an 11mm MAS 1873 Revolver (and ammunition) that was confiscated by police during the 2011 London riots. A gang in the West Midlands city of Birmingham were filmed trying to shoot down a police helicopter with this, a FEG PA63 and two other pistols.
I can’t see the calibre marking in the headstamp, but can make out that it is Starline Brass. Starline doesn’t list this calibre in its range, so the brass must have been modified from another calibre. The reloads look too well made to have been made by the gang members. What case is commonly used to convert to 11mm French? They do look to be possibly rimless cartridges in the photo. Would a .45 ACP chamber in a MAS 1873? I understand that even if it would chamber, no one with any common sense would try firing it (higher pressures and larger bullet diameter). The chambers could also have been poorly drilled or filed out of course.
It is odd that they would use this gun when our cities are awash with illegal semi-auto pistols in the usual calibres. The only explanation that I can think is that the revolver and cartridges were stolen from a legitimate shooter before the 1997 handgun ban.