Here's some "gun history" that might be of interest .... Prior to the Civil War, the most common handgun was a black powder single shot pistol. As technology advanced and revolvers were invented, gun manufacturers noted an interesting phenomena. A pistol with a non-vented barrel (measured from breach face to muzzle), delivered the same velocity as a revolver with a barrel of the same length that was measured from the front face of the cylinder to the muzzle. Primarily a sales gimmick, revolver barrels were measured from the cylinder face to the muzzle. This leveled the playing field between a pistol and a revolver, making muzzle velocity the same, even though they were measured differently. Seems there's always a reason .....
Later when modern cartridges were invented, the same concept applied. Turns out the B/C gap in a revolver vents quite a bit of pressure, which robs considerable muzzle velocity. An excellent example is a 45 ACP, which is chambered in both pistols and revolvers. In a 1911 with a 5" barrel, there is about 4" rifled barrel. In a 5" barreled revolver chambered in 45 ACP, the cylinder is about 2" long and the cartridge occupies about an inch, leaving about an inch of cylinder throat plus a 5" barrel for a total of 6" of bullet travel. So, the B/C gap loss makes the revolver chronograph very close to the same as an "equal length" pistol, assuming they are both measured properly. You can prove this concept by chronographing a 5" 1911 and a Ruger Blackhawk with a 4 5/8" barrel. With GI ball ammo (230 gr FMJ), the 1911 will chrono at about 850 fps, whereas the BH will chrono at about 830 fps with the same ammo, even though the BH has 1.37" more bullet travel. I measured my Ruger SR1911 barrel at 5.050" and my 45 BH at 6.42" (from breach face to muzzle) to get these actual numbers and have chronographed them side-by-side. I have also tested a 9mm 4 5/8" BH and a Beretta 92FS and got identical results.
B/C gap loss is about 1.5% for each .001" gap, no matter what cartridge is used ... so, an average revolver with a .006" B/C gap will lose about 9% of its muzzle velocity compared to an equal length non-vented barrel. The math works out where the B/C gap loss is compensated by the gain of the added bullet travel. So in a nutshell, pistols will chronograph virtually identical to equal length revolvers when shooting the same ammo.
The Gun Control Act of 1968 made some defined changes. First, all handguns used to be called pistols. BATF began requiring manufacturers and importers to report all guns manufactured or imported by type ... rifles, shotguns, pistols and revolvers. Pistols are defined as any handgun without a revolving cylinder. Obviously, revolvers are all handguns with a revolving cylinder. Along with these "official definitions" came the "official" way to measure barrel lengths. All rifles, shotguns, and pistols are measured from the muzzle to the breach face as had been the tradition for well over 100 years. Revolvers are measured from the muzzle to the cylinder face, which has also been tradition since revolvers were invented. If you look at Ruger's web site (or any other gun manufacturer), they always separate the two handgun categories ... pistols and revolvers.
There you have it ... Gun History 101. There's a lot more but I don't want wingspar to get mad about being off topic.