Firearm

A flintlock is a type of gun ignition mechanism that was 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.

Overview
The flintlock became a symbol of its day, and remained in common use for about 200 years until the introduction of new ignition types, such as the percussion cap system, during the 19th century. It was still found in use even after being out-dated by newer ignition systems in wars, such as in the early days of the American Civil War (especially with Confederate forces). The flintlock is still favored by traditional gun powder ("black powder") shooting enthusiasts, and is kept in production for that reason. The flintlock rifle had spiral grooves cut in the bore that greatly improved accuracy and range, but could take several minutes to reload. The slender flintlock rifle of the colonial American frontier has an iconic place in American history. However, most flintlock firearms were smooth-bore and fired either a large lead ball or were used as a shotgun — often called a Fowler.

The flintlock was renowned for becoming next to useless in the rain, but was a great improvement over the matchlock which required a lit slow-burning match cord that was impossible to keep lit in the rain. In damp weather, shooters would keep a cover of some sort over the lock to keep it dry until use. Most flintlocks had only one barrel and had to be reloaded from the muzzle after each discharge. Civilians typically used a powder-horn and ball bag to reload after each shot. In military use, soldiers used preload paper cartridges that contained both a measure of gunpowder and a ball (bullet) to speed up reloading. Due to the length of time it took to reload, most stationary gunmen using the weapon would carry several spares (often pistols) 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 was typically a military version of a flintlock. It was a smooth bore weapon into the early 19th century. Like all muzzle-loaders, it required a tedious reload, however cartridges (a paper tube pre-filled with powder and ball) sped up the process. A trained soldier could fire from 3 to 5 times a minute and carried several dozen cartridges in specially designed containers slung over the shoulder or on a belt about the waist. The flintlock musket was in use from colonial times into the first half of the 19th century. A rifled musket was a later military firearm that had spiral grooves in the barrel. The percussion cap rifled musket was the standard issue military firearm by the mid 19th century.

One Piece
The flintlock is the main gun type seen in the series. They are much 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. Most of the flintlock weaponry in One Piece are also semi-automatic, something only found on a few exotic multi-barrelled volley gun designs in real life. Both Yasopp and Lucky Roo, along with many background characters, have been seen using these type of guns. The guns, however, still fire small, round, ball-like bullets like those used by the traditional guns until the last couple of centuries.

Types of Flintlocks
Because of the more advanced structures of the standard flintlock, there are other models and ammunition developed for use. Also, even standard flintlocks may be customized in physical appearance to suit one's motif.
 * Flintlock .44 caliber 6 shot revolver (フリントロック式44口径6連発リボルバー): A new model from the South Blue, capable of rapid-fire with six chambers. Used by Mr. 5, who provides ammunition via his Devil Fruit powers.
 * Yellow Gun (黄色い銃): A pistol shaped like the number "7", and is designed to fire dice-shaped bullets that explode on impact. Used by Mr. 7.
 * Gero Gero Gun (ゲロゲロ銃): A pistol with a muzzle shaped like a frog's head, and fires frog-shaped bullets that explode on impact. Used by Miss Father's Day.
 * Flash Guns (閃光銃（フラッシュガン）): Dual-barreled flintlocks with ornate design, and Flash Dials installed within to hide the shots with a flash. Braham uses two of them.
 * Some flintlocks are installed with dual barrels, allowing two rounds per shot. Krieg wields two of them while Caesar Clown wields one.
 * World Nobles wield gold-plated single-barreled pistols. Aside from its more grandiose-color, it functions no different than the standard flintlock. Saint Charloss wielded such firearm, and (in the anime) so did Saint Mjosgard.
 * Donquixote Doflamingo wields a single-barreled, obsidian black pistol with luxurious gold plating. He loaded it with lead bullets to inflict lead poisoning.

Other Types of Guns
Aside from the standard flintlocks and muskets (along with their modified variants), there are other type of guns introduced in the series, though they appear rarer than the flintlock. Most of their designs are based off of the flintlocks, bearing the appearance and mechanism that fits the older times.
 * Gin's shotgun, containing dual-barrels.
 * Krieg's Needle Machine Gun, firing sharp stakes at a rapid rate.
 * Mr. 4's giant gun, which bears resemblance to a hand-cannon or bazooka, and "fed" a Zoan-class Deivl Fruit.
 * Van Augur's special rifle, Senriku, possesses tremendous range.
 * Franky's machine-guns, amongst his built-in weaponry.
 * Duval's harpoon guns, loaded with scorpion-venom laced ammunition.
 * Roswald's cane gun.
 * Jalmack's bazooka, which takes the form of a gigantic rifle and equiped with a cross-hair.
 * Caribou, Coribou and Baby 5 all wield Gatling guns, the former uses a crank mechanism.
 * Gladius has firing mechanisms equipped on his arms to fire bullets powered by his Devil Fruit powers.

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.