An unpopular opinion, if I may. Anyone shooting an out-of-proof British-made gun is asking for trouble. Maybe not today, or tomorrow, but at some point failure will happen. Maybe not to the OP, but perhaps to the person he sells it to in future, or to an unlucky inheritor. Someone who might not know much about pressures, and has some great 'hot' loads to try. There is a reason why Britain forbids the sale of out-of-proof guns, and plenty of reasons why proof laws exist -- do you really want to trust your eyes and fingers with a for-profit company? As Canvasback said, American makers, not protected by proof laws (protection goes both ways), made their barrels excessively strong (and heavier), and can therefore suffer a bit of bubba-ing, professional or amateur.
On this gun, which looks very nice by the way, the damage is done. Whoever had the work done has likely fired 3" shells in it; firing more 'test' loads isn't going to prove anything. The question is, have the modifications and previous use taken it into dangerous territory? Only a competent double-gun smith with the right measuring tools can tell. Along with the chamber lengthening, the bores may have been further honed. That is always an issue with modified guns, there may be other changes that are not readily apparent, you just can't know.
The OP might want to get his hands on a copy of the book 'Trade Guns of the Hudson Bay Company 1670-1970' by S. James Gooding. His gun was likely made by one of the following makers: Isaac Hollis; Bentley & Playfair; C.O. Bircham; C.G Bonehill (Ltd.); W.J Jeffrey & Co.; Thomas Bland & Sons; JP Clabrough (and later JP Clabrough & Johnstone); or Joseph Bourne & Son. The Imperials were made in great quantities to special order for the Hudson's Bay Company, while others were probably taken from the manufacturer's inventory and stamped with a brand name, explaining the wide variety of types and gauges under the Imperial banner.
On this gun, which looks very nice by the way, the damage is done. Whoever had the work done has likely fired 3" shells in it; firing more 'test' loads isn't going to prove anything. The question is, have the modifications and previous use taken it into dangerous territory? Only a competent double-gun smith with the right measuring tools can tell. Along with the chamber lengthening, the bores may have been further honed. That is always an issue with modified guns, there may be other changes that are not readily apparent, you just can't know.
The OP might want to get his hands on a copy of the book 'Trade Guns of the Hudson Bay Company 1670-1970' by S. James Gooding. His gun was likely made by one of the following makers: Isaac Hollis; Bentley & Playfair; C.O. Bircham; C.G Bonehill (Ltd.); W.J Jeffrey & Co.; Thomas Bland & Sons; JP Clabrough (and later JP Clabrough & Johnstone); or Joseph Bourne & Son. The Imperials were made in great quantities to special order for the Hudson's Bay Company, while others were probably taken from the manufacturer's inventory and stamped with a brand name, explaining the wide variety of types and gauges under the Imperial banner.
