WWG 1st Edn 1881 refers to chamber lengths only as - "12 gauge and larger should be 2 5/8", 2 7/8" or 3 1/4" - better ballistics for heavier than standard pigeon/trap loads being produced by brass case guns, compared to a falling off in pattern quality in paper case guns with extended chamber length. (I guess that 3 1/4" refers to 8 G only at this date.)
As Joe points out there was an acknowledged problem to accomodate heavy loads and incorporate adequate length of wadding column in standard chamber length cartridge cases (hulls).
Presumably TT's Scott could be either 2 5/8 or 2 7/8" , if not actually 2 3/4" - as original or lengthened at some stage.
Boring of .727" would suggest a paper case gun rather than a brass case gun ( and if built actually in 1872 this date would likely be too early for it to be a designated brass case gun ).
As usual, more information provided on proof marks may help to indicate the primary design and intended quarry/use of the gun.
With regard to a date for introduction of 2 3/4" chambers it may be helpful to look through a few of the early catalogues of the ammunition manufacturers.
