A clip is a device that is used to store multiple rounds of ammunition together as a unit, ready for loading directly into the magazine of a repeating firearm. This speeds up the process of loading the firearm as several rounds can be loaded at once, rather than one round being loaded at a time. Several different types of clips exist, most of which are made of inexpensive metal stampings that are designed to be disposable, though they are often re-used.
At a later point in time, methods were devised to use stripper clips for the loading of detachable box magazines. This method generally requires the use of a disposable guide which connects the clip to the magazine. Both the clip and guide must be discarded before the magazine can be attached to the firearm.
The major shortcoming of the en bloc design that prevented further adoption is that it is typically difficult, or in some cases impossible, to "top off" or refill a partially-loaded magazine without first cycling all unfired rounds through the action.
The difference between a moon clip and a speedloader is that the cylinder may be closed with the moon clip attached, whereas the speedloader must be removed before the cylinder can be closed.
Moon clips were devised during World War I to allow ammunition designed for semi-automatic pistols to be used in revolvers, simplifying logistics. Revolvers are designed to use rimmed cartridges. As such, their extractors are incapable of removing the rimless cartridges that semi-automatic pistols use. Moon clips are made from spring grade steel, and hold the cartridges firmly enough that a special tool is generally used to insert and remove them. The clips remain in place during firing, and after firing, are used to extract the cartridges.
Moon clips, in particular, may be even faster to use than a speedloader. Jerry Miculek, a top IPSC revolver shooter, has demonstrated the ability to fire six shots from a .45 ACP revolver, reload, and fire six more shots to the 6x11-inch A zone of an IPSC target at fifteen feet in under three seconds, including reaction time to the start signal. This feat was possible by using moon clips to allow quick and reliable ejection of the fired rounds, and a quick reload of all six chambers at once.