Firearm

A flintlock was a type of gun loading mechanism first introduced in about 1630. It was a revolutionary design that replaced the firearm-ignition technologies at the time, such as the matchlock and wheel-lock.

The gun became a symbol of its day, and in common use lasted for about 200 years until the introduction of new gun types, such as the percussion cap system, during the 19th century. It was still found in use even after being out-dated by newer gun loading systems in wars, such as in the early days of the American Civil War. The gun is still favored by traditional gun powder shooting enthusiasts, and is kept in production for that reason.

The flintlock was renowned for becoming useless in the rain, and most versions had enough rounds to fire only once before reloading. Due to the length of time it took to reload, most gunmen using the weapon would carry several spares on them so they did not have to waste time reloading after each round. However, despite its downfalls, the flintlock was a favorite military weapon.

Musket
A musket is a rifle version of a flintlock. Like a flintlock, they require tedious reload. However, they have a longer range. It was used in colonial times.

One Piece
The flintlock is the main gun type seen in One Piece. They are slightly more advanced than the traditional flintlocks of real life, in that they do not suffer the same reloading time and limited rounds as found in the real life versions. Both Yasopp and Lucky Roo have been seen using these type of guns as well as many background characters. The guns, however, still fire small, round, ball-like bullets like those used by the traditional guns until the last couple of centuries. The most open use of a Flintlock comes from Mr. 5 who used a Flintlock .44 caliber 6 shot revolver, a new model from the South Blue in combination with his Devil Fruit powers.

Other type of guns have been seen, such the machine gun found amongst Franky's built-in weaponry; however, they are rarer than the flintlock. The guns used by the World Nobles are also different in appearance, as the one used by Saint Charloss is golden and more advanced-looking.

Other Types of Guns
In the series, there are other types of guns used, and all of their design seems to be based off of the flintlocks, bearing the appearance and mechanism that fits the older times.

Translation and Dub Issues
4Kids reaction to the presence of the guns is to color them green (or blue in a few cases), remove them, or turn them into something else (hammer-gun, arrows, Super Soakers, or cork gun). However, it should be noted this is a common edit with all guns, not simply flintlocks.