A Boring Dystopia

18th century artillery? I suppose it also depends on whether we were talking a squad, platoon, etc., whether the single person has to defend a particular area like their home with family inside, whether this whole army was in one place, whether the army intentionally traveled through time vs. simply were teleported there against their will (in which case they likely could be convinced to surrender, especially upon seeing a helicopter - and like, wherever they came from were they a defensively minded unit, like would their morale be crushed to realize that neither victory nor defeat would ever allow them to see their loved ones again - plus these defenders could literally be their own family members as in descendants), etc.

But without going into such details, my point was they technology has advanced quite a bit since the 18th century, which would give someone with a modern gun a decided advantage for them to make use of.