It looks to have an adjustable, possibly set trigger. This is normally found on rifles rather than muskets which usually weren’t accurate enough to benefit from such devices. The octagon barrel is also more common to rifles than muskets. Very nice looking piece, W Chance & Son was a Birmingham England firm with a history of making and exporting trade guns to NW US. This is a fine quality sporting rifle, probably from the 1840-1860 period, not a trade gun.