That would initially suggest William Moore & Grey, a very good London maker.
However ...my understanding is that their name was used spuriously, often by a Belgian maker. As far as I am aware they always signed their guns 'Wm Moore & Grey'. They also always numbered their guns. So you need to be wary of attributing this gun to them until further investigation has been carried out.
It would be helpful if you could kindly post some photos of the gun, especially the markings and proof marks. We may then be able to assist further.
Tim
