The .45 BPM was originally developed for use in replica Colt Walker black powder revolvers. Specifically, Colt Walker replicas that have been converted to allow the use of firearm cartridges by incorporation of a "converter cylinder". A. Uberti Firearms is one such company that manufactures replica revolvers.
The Colt Walker was the largest and most powerful black-powder revolver produced prior to 1970. Because its cylinder could hold 60 grains of black powder in each of its six chambers it is regarded as the world's first magnum revolver. The Colt Walker was unsurpassed in power by any commercially-manufactured repeating handgun from its introduction in 1847 until the arrival of the .357 Magnum in 1935. Samuel Colt, with suggestions from Captain Samuel H. Walker, designed it as a "cap and ball" revolver to shoot both lead round balls and picket bullets. This was prior to the development of self-contained cartridges as used in most modern firearms today. As such, an unconverted Colt Walker is a front loading percussion revolver.
The .45 BPM uses the .460 S&W Magnum brass casing as its container. The .460 S&W Magnum was derived from the .454 Casull while the .454 Casull was derived from the .45 Colt. Because of this lineage the .460 S&W Magnum has similar dimensional attributes to the .45 Colt with the exception of a much longer casing. As such, revolvers chambered to .460 S&W Magnum will usually chamber the shorter .45 Colt. The reverse, however, is not true as most .45 Colt revolver cylinders are not long enough to chamber the .460 S&W Magnum cartridge. Two examples of the exception to this rule are the Colt Walker and the Taurus Judge. However, it is standard practice to rebate the inside of a revolver's chambers only to a depth equal to the targeted cartridge's length. This prevents either accidental or intentional loading of longer but unsafe cartridges in firearms not designed for or tested for more powerful cartridges.
After the introduction of the .45 Colt handgun cartridge, in 1872, the conversion of .44 caliber percussion revolvers began. Percussion cylinders were replaced with rear loading cartridge cylinders. This allowed the use of the .45 Colt cartridge. The load range for this cartridge was typically 28 to 40 grains of black powder. Thus, a Colt Walker revolver converted to shoot the .45 Colt cartridge is limited to a maximum of 40 grains even though the revolver was originally designed to load up to 60 grains. The .45 BPM cartridge was developed to allow for black-powder loads of more than 40 grains. This gives the converted Colt Walker the convenience of using self-contained cartridges and the advantage of loads beyond the 40 grain limitation of the .45 Colt cartridge and provides the added benefit of moving the bullet closer to the forcing cone as compared to the .45 Colt. This can potentially improve accuracy by reducing "bullet jump".
The .45 BPM can hold up to 60 grains of black powder. A 60 grain cartridge configuration may be desirable to use as a novelty rifle cartridge such as for a revolver carbine.