In 1860s America the percussion revolver was the prominent fighting handgun. The 44 caliber, or “44/100ths calibre” was so named at the time because of the gun’s bore. Today we tend to use groove diameter to define caliber, but then why does a modern 44 use a .429″ bullet and a 38 use a .357″ bullet?
Much of the answer lies in this one bullet. When the 44 caliber percussion revolver was converted to fire metal cartridges, it presented the following challenges. The cartridge case of course had to fit into the percussion cylinder chambers, and had to fire a bullet of around .452″ to fill the grooves in a 44 caliber bore. SO the metal case had to fit inside a .452″ or so chamber, and fire a .452″ or so bullet, AND therefore the bullet had to have a heel base of around .429″ to fit inside the metal case. Such is the 44 Colt cartridge. It was built for cartridge conversions of percussion cylinders. It’s a 44 caliber because of the naming convention of the time which went by bore, rather than groove diameter, it uses a .452″ bullet and has a .429″ heel to fit in the case.
From that transition cartridge we see the seeds of how a modern “44” came to have a .429″ bullet. A similar thing occurred with the conversion of 36/100ths calibre percussion revolvers, and that’s how a 36 used a .380″ bullet and how a modern 38 uses a .357″ bullet.