You question might need to be refined a bit ( could you post a photo of a specific example?), but plenty of guns used by the military after 1700 were not equipped to have bayonets. The Algerians used guns without bayonets in their resistance to the French in the early 19th c., but that's because much of the time their tactics tended towards quick raids, and so their troops were on horseback - where bayonets were useless. European cavalry didn't bother with bayonets, either.