A powder flask is a small container for gunpowder, which was an essential part of shooting equipment with muzzle-loading guns, before pre-made paper cartridges became standard in the 19th century. They range from very elaborately decorated works of art to early forms of consumer packaging, and are widely collected. Many were standardized military issue, but the most decorative were generally used for sporting shooting.

Although the term powder horn is sometimes used for any kind of powder flask, it is strictly a sub-category of flask made from a hollowed bovid horn. Powder flasks were made in a great variety of materials and shapes, though ferrous metals that were prone to give off sparks when hit were usually avoided. Stag antler, which could be carved or engraved, was an especially common material, but wood and copper were common, and in India, ivory.

Many types of early guns required two different forms of gunpowder (such as a flintlock with finer priming powder for the pan, and a coarser standard powder for the main charge), necessitating two containers, a main flask and a smaller priming flask.

Apart from the horns, common shapes were the Y formed by the base of an antler (inverted), a usually flattened pear shape with a straight spout (poire-poudre or "powder pear" is a French term for these), a round flattened shape, and for larger flasks a triangle with concave rounded sides, which unlike the smaller flasks could be stood upright on a surface. Many designs (such as horn and antler types) have a wide sealed opening for filling, and a thin spout for dispensing. Various devices were used to load a precise amount of powder to dispense, as it was important not to load too much or too little powder, or the powder was dispensed into a powder measure or "charger" (these survive much less often). As early as c. 1600 a German flask had a silver spout with a "telescopic valve, adjustable for different sizes of powder charges".

Although forms of pre-packed paper cartridges go back to the Middle Ages, these were for several centuries made up by the shooter or a servant, rather than being mass-produced, requiring a container for the gunpowder, which came loose. Unlike modern cartridges, these were not inserted into the gun themselves, but were rather a pre-measured amount of powder stored in a paper wrapper, sometimes with the ball included as well. Loading the gun involved tearing open the package, emptying the powder into the muzzle and pan, inserting the ball with the paper doubling as wadding, and then ramming home the charge. This was somewhat faster and more convenient than measuring out a powder charge each time, especially in a combat situation. However, there was no large-scale manufacturing of these cartridges until the 19th century, and even then the benefits mostly lay with military use; the added cost made them less popular with civilian shooters until the advent of the self-contained metallic cartridge and the breech-loader.

While loading a muzzleloader, an important safety concern was that when reloading a muzzle-loading gun soon after a shot there might be small pieces of wadding burning in the muzzle, which would cause the new load of powder to ignite as a flash. So long as no part of the loader faced the end of the barrel this was not likely to lead to serious injury, but if a spark reached the main supply in the powder flask a dangerous, even fatal, explosion was likely. General Sir James Pulteney, 7th Baronet, was one such victim; he died in 1811 from complications after losing an eye when a powder flask accidentally exploded in his face in Norfolk. Charles Kickham, prominent in the Irish Republican Brotherhood, grew up largely deaf and almost blind as the result of an explosion when he was 13, in about 1840. Various precautions were taken in the design and use of powder flasks to prevent this from happening, and expensive examples from as early as the 16th century usually have springs to automatically close the dispensing spout (this is much less common with the cheaper horn type).

Modern manuals on muzzle-loading guns all say the flask should never be used to pour powder directly down the muzzle, to avoid dangerous overcharging and possible burst barrels, but from the English sporting press of the 18th and early 19th centuries, it is all too clear that this was then common practice, resulting in many accidents. Some YouTube videos demonstrating loading maintain the old traditions. Instead, the powder should be poured into an intermediate container known as a charger or powder measure. Sometimes, the cap to the spout represented the measure, especially for priming flasks. Sometimes, the spout itself was the measure, with a sliding device to shut off the supply at the base, as well as a cap. This type became the norm in the mid-19th century.

High-quality guns would often have come with a matching flask, chargers, and other accessories. Many flasks have small rings for a cord, which was slung around the neck to carry them, especially before large pockets on hunting clothes arrived in Europe in the 18th century. Some examples have original elaborate cords with knots and tassels.