The humble beginnings...

of firearms technology looked a lot like this:

This is a fully functional, reproduction arquebus. The earliest hand-carried firearms were not much more than little cannons on sticks (or with hooks to attach to battlements). In terms of firepower, they offered little advantage to longbows in terms of distance, accuracy, and rate of fire and little advantage to crossbows in terms of accuracy and penetration power. Their value, however, was that they were cheaper to make and required much less training to use.

This example is very similar to those depicted on the walls of defensive castles in and around 1470, and has only two moving parts; a hinged flash pan cover to keep the priming charge more or less dry, and the "serpentine" - an early precursor to a mechanical trigger. The serpentine held a "match" which was basically vegetable twine soaked in saltpeter to encourage slow burning. The match was lit for the entire duration of the battle, or until the match was burnt. As you can imagine, the need to keep the match lit hampered foul weather operations, encircled the arquebusier in constant smoke and made his glowing match visible at night.

With all of the negative attributes of this sort of firearm, it's a wonder the technology ever advanced. Bert Hall gives an excellent account of the changing weapons and tactics in Renaissance Europe in his book Weapons & Warfare in Renaissance Europe. He makes the case that all of the challenges to firearms (hand held) use are obviated if the guns are used in a fortification/castle setting, where powder and matches could be kept dry, the slow process of muzzle-loading the guns could be done under cover, and less exposure of the shooter to enemy fire would be possible. Coupled with parallel changes mediated by gunpowder improvements and the increasing use of larger and more powerful cannons, this was just the advantage the technology needed to advance and become commonplace.

With a few years, handgun (meaning man-carried) technology advanced from arquebus through matchlock, wheellock, dog log, and flintlock, which remained on the scene for a few hundred years until percussion came along. For an interesting expose of the developments of this later period, track down the book The Flintlock: Its Origin and Development by Torsten Lenk. It's older, but worth a few bucks just for the illustrations alone.