Podewils, the proofmarks show indeed the rifle was made/proofed after April 1,1893, the day the 1891 prooflaw became effective. The eagle is the mark for the preliminary proof of the rough barrels, searching for defective material.The G under crown stands for "gezogen" = rifled, final proof charge of the rifled barrel fired. "U under crown" is the same as the British "view" mark, it stands for "Untersuchung",the thorough inspection of the gun after firing the proof charge. The "S in diamond" is probably the mark of the unidentified barrel maker. "46" is indeed the gauge number, lead balls to the pound, same as in Britain. It tells us a 46 gauge plug = 11.84mm = .466" passed the bore at the proofhouse, a 45 gauge = 11.91mm = .469" did not, according to the proof tables. This is for the bore, not the rifling or bullet diameter! The gauge numbers went out of use in 1912, mm numbers used from then on. A cartridge fitting your dimensions is not listed even by Dixon, so it must be a very rare wildcat or proprietary chambering. IMHO your rifle was made shortly after 1893, very close to the end of the blackpowder era.