Several have posted here about single-barrel hammer guns, and I'm with them on that. My ideal is, like most things in life, a compromise. It is a 16-bore made by Thomas Seymour of London (1841-1844), converted from percussion to centre-fire some point in the 1870s. The twist barrel was re-proofed for smokeless powder at some point, and it weighs just under 5 pounds. OK, the non-rebounding lock and the Jones underlever are not very practical, but what a joy to carry a bit of history in the field! Most importantly, it is still taking grouse.
