Hi, Antenna,
Actually there is no such round as the .45 Long Colt. The correct name is just .45 Colt. But folks wanted to distinguish between it and the .45 Automatic Colt Pistol (ACP) cartridge and took to calling the older and longer cartridge the .45 "Long" Colt.
The .45 Colt was developed in the early 1870s for the Model 1873 Colt single action revolver that was adopted by the U.S. Army. Colt later made the gun is many other calibers, but the first and (I think) most common was the .45, primarily because that was the service cartridge.
The .45 Colt is a good, accurate cartridge, but has never been "souped up" by the factories because of the many older guns chambered for it and because of the weakness of the older cartridges. With modern cartridges, a modern revolver and good reloading, it can approach the .44 Magnum.
One of the drawbacks of the .45 Colt is the small rim, which was necessary to get six cartridges in a cylinder within the limitations of the old Colt Navy frame, which was the basis of the 1873 SAA. Since the rim was not needed for extraction in a single action revolver with an ejector rod, its size was not too important, but it makes reloading a little harder.
Jim