The guns of the Gunpowder Age are generally some variant of the arquebus, matchlock, flintlock, or musket (the terms are generally interchangeable). The key early innovations in that era were the development of the triggered mechanism (originally a smoldering piece of rope, aka a match, later a piece of flint that sparks when struck), the stock for stability, and the long barrel for sighting.
The developments that led since then to the modern gun are breech loading, the Minié ball, magazine-fed ammunition, paper and later metal cartridges, rifling, smokeless powder, and the automatic firing mechanism. With the exception of the paper cartridge, all of these technologies were developed in the latter half of the 19th century.
Simply put, pre-American Civil War guns were tactically very different in operation from 20th and 21st century guns. As an aside, this is why WWI was so bloody: generals were trying to fight a war with 20th century guns and 18th century tactics.