Green’s Dictionary of Slang

bess n.2

[abbr. brown bess n.]

a firelock or musket.

[Ire]Dublin Eve. Mail 30 Aug. 4/4: [from Times (London)] The musket-rifle (improperly called ‘Minie’) being constructed on more scientific principles, ‘fairly beats poor Bess out of the field’ .
[UK]Hotten Sl. Dict.