I suspect this belonged at one time to the UKicon Boys Brigade, who used "deactivated" MH Carbines for doing drill in the 1890s.. We have some pictures of the BBs in our village with them in around 1900.

The Boys Brigade was a church based youth organisation based on military lines.

I have seen these reactivated to use .577/.450 by TIG welding up the slot, although one I am aware of had a groove left across the chamber that would catch cases.. I would not be happy sleeving one to .303! These barrels were often made from wrought iron and were never designed for nitro pressures. Conversion was always by barrel replacement with proper proof tested steel barrels. They are usually Metford rifled and are often not good at stabilising modern bullets!

I would be tempted to try and reinstate it to the original calibre.. You can make .577/.450 cases out of brass shotgun cases, and Lee make the dies. You need to find a good TIG welder, and use a copper heat sink in the chamber.
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