People often make too much of a deal about the "Code Duello of 1777." Even in the U.K., many duels were fought outside of these guidelines, and practices changed as time went on and pistols improved. Taking careful aim was considered dishonorable early on for example, but over time aiming, better sights, and set triggers became the norm. Single set triggers that push forward to set like those on the Hamilton-Burr pistols were the norm for a while not generally a secret, but better non-set triggers became the norm as time went on. Technically, any weapons could be used in a duel. There are recorded formal duels that were fought with rifles, blunderbusses, billiard balls, etc. though those aren't really the norm. Dueling pistols from the U.K. and in the U.S. were generally smoothbore in keeping with the "Codes of Honor," but some have faint "scratch rifling" or rifling that is hidden by stopping before the muzzle. As long as both pistols used are equal, that isn't really cheating. In the case that each participant brought his own set and they were supposed to be smoothbore, hidden rifling would be cheating.
While dueling faded by 1850 in the UK, in Continental Europe it continued, and by the time of the percussion system was in wide use, rifled pistols were preferred for duels, and their rifling was not hidden. One way to tell target pistols in this period from dueling pistols is that the target pistols often have adjustable rear sights unlike this pair which has fixed sights. That said, most "dueling pistols" by the mid-1800s probably saw more use as target pistols in shooting parlors.
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