Nitrox - very interesting link, using nitro powders a 2 3/4 inch case could be loaded to equal the 3 inch for all practical purposes

Caprivi - as the date of manufacture is uncertain, if later - 1890s, it could have been built for nitro, (I believe the .577 Nitro express was introduced around 1898 by H&H?) the Nitro Proof did not come into use until 1904, IIRC??
[BTW when H&H examined my ball gun their only criticism was that the later nitro proof was unnecessary - especially in ball guns & rifles!]

Gatsby - Barrel length is 26 3/4 inch, weight of barrels 7 lb 5 oz, total weight 12 lbs 2 oz (not 12 lbs 5 oz as I posted)

TH44