Howdy
Smith and Wesson made lots and lots of small 'Pocket Pistols'. Here is a photo showing some comparative sizes. The three revolvers on the left are what are known as Tip Up revolvers. These were the first revolvers S&W made, starting in 1857. There was a latch at the bottom of the barrel, and a hinge at the top of the barrel. The little brass framed Tip Up at the bottom left is called a No. 1. This particular one is a #1, 1st Issue, 5th Type and it left the factory in 1859. It was a seven shot revolver shooting what we would call today the 22 Short. Next up is a # 1 1/2 32 Rimfire five shot revolver, and the big Tip Up is a #2 Old Army 32 Rimfire six shot revolver. The #2 was popular with Union officers in the Civil War. It was the largest Tip Up S&W made, they experimented with a 44 caliber Tip Up but the design was not strong enough for a 44 caliber revolver. The #2 with its standard 5" or 6" barrel was fairly large and not easily concealed. Over on the right are Top Break revolvers. Unlike the Tip Ups, they were reloaded by lifting the latch at the rear of the barrel and rotating the barrel down. The large screw under the barrel is the pivot point for the barrel. S&W started making Top Breaks in 1869 when the White patent, which they controlled, for boring through the cylinder for cartridges, expired. From the top, a big #3 2nd Model Russian, six shots chambered for 44 Russian which shipped in 1875. Much too big to be easily concealed, this is a belt gun. Next down a 38 Single Action 1st Model, this model is often called the Baby Russian, a five shot 38 S&W revolver, it shipped in 1876. And finally, a 32 Single Action 5 shot 32 S&W revolver which shipped in 1889. Notice all of these revolvers have external hammers and none of them are safe to carry fully loaded. A strong blow to the hammer would probably discharge them, even it the hammer was set at the fragile 'safety cock' notch. The Tip Ups are very simple revolvers, there is no 'safety cock' or half cock position for the hammer, it is either at full cock or all the way down. So the firing pin of a Tip Up is resting directly on the rim of a cartridge under the hammer. The Top Breaks all have a 'safety cock' notch, S&W actually called it half cock, because the hammer was either all the way down, about 1/8" back at 'half cock' or fully cocked.
Now here is an interesting point. After the Civil War many towns across the country began enacting laws against open carry of pistols. Up to this time S&W had only been making the tiny #1 Tip Ups and the larger #2 Old Army. In order to sell a revolver that would be more easily concealed, in other words a Pocket Revolver, they came up with the # 1 1/2 Tip Up. Bigger than the #1, but smaller than the #2. Like the #2, the #1 1/2 was a 32 Rimfire revolver, but because the frame, and hence the cylinder were smaller then the #2, it was a five shot revolver.
I suspect most did not carry these revolvers with an empty chamber under the hammer. I have no proof of that, but that is my suspicion.
Here is how a Tip Up worked. This Old Army has its barrel rotated up and cylinder removed, ready to be loaded with six fresh 32 Long Rimfire cartridges.
I find the term 'vest pocket' pistol very misleading. Here is the little S&W #1 pictured with a 25 ACP Browning 'Baby' Auto. I would consider the Baby Browning to be a 'vest pocket' pistol, but the vest would have to have pretty big pockets.
I would consider all of these revolvers to be Pocket Pistols. Notice there is quite a variation in size. Top left is a S&W 32 Safety Hammerless Bicycle Revolver. A five shot revolver chambered for 32 S&W. These short barreled models were known as Bicycle Revolvers because at the turn of the Century riding bicycles was becoming a popular national pastime. Easily hidden in a pocket in case Daisy had to be protected from ruffians while riding the bicycle built for two. This one shipped in 1908. To the right is another Safety Hammerless, this one is a 38 Safety Hammerless, 3rd Model, it shipped in 1896. The Safety Hammerless revolvers were specifically designed to be carried in a pocket. Double action only, they were meant for close quarters defense, not target shooting. The lack of an exterior hammer (there was an interior hammer hidden under the side plate) meant they would not snag on fabric when being drawn from a pocket. And all S&W Safety Hammerless revolvers had a grip safety, not much different than the grip safety on a Colt 1911. Completely safe to carry fully loaded, the trigger would not move unless the grip safety was depressed. Next down is the S&W #1 1st issue. Below that to the left is the classic Remington Double Derringer. Two shots, 41 Rimfire. Not much point in only carrying it loaded with an empty under the hammer. To the right of the Remington is a Sharps Model 1, sometimes called a Pepperbox. Four barrels, four shots, 22 Short only. At the bottom is a Colt five shot 31 caliber 1849 Pocket Cap & Ball revolver. With it's 4" barrel, Colt considered it a Pocket Revolver and so do I. No 'safety cock' notch on the hammer of any Colt Cap & Ball revolver, only half cock for loading and full cock. This one does have small pins between the nipples and a hollow on the hammer to receive the pins, but the pins and hollow are worn away and will not keep the hammer between chambers. When it was new, it could be completely loaded with five balls and the hammer placed between the nipples.
Regarding letting the hammer down on a Cartridge Colt between chambers: While this would work with a Cap & Ball revolver that had pins between the chambers, or notches to receive the hammer nose, it will not work with a large caliber Colt Single Action Army. I have tried it. Here is a 2nd Gen Colt cylinder with five spent 45 Colt rounds in the chambers. Note how little space there is between the rims at the narrowest point. If the hammer is let down so that the firing pin rests between chambers, it is resting right at that narrowest spot. That is the geometry of the cylinder and the hammer. Notice the shape of the firing pin and how rounded the tip is. With the firing pin between chambers, the slightest effort to rotate the cylinder will result in the firing pin riding up and over the bevel of the rims. It will not stay between the rims. This might work with smaller cartridges, such as a 38 Special, where there would be more space between the rims, but it cannot be relied on with a Colt chambered for large cartridges such as 45 Colt, 44-40, or 38-40.
Do I carry a Colt with the hammer down on an empty chamber? You bet. I have heard and read about too many instances of men being accidentally shot when their Colt fell to the ground and landed on the hammer. Even a stirrup falling on the hammer while preparing to mount a horse has discharged the revolver and shot the rider in the leg. Don't forget, Ruger started putting transfer bars in all their revolvers in the 1970s after they lost some expensive law suits by shooters who did not follow the safety rule of only loading the old Three Screw Rugers with five rounds and leaving an empty under the hammer. The old Three Screws worked just like a Colt, there was no transfer bar.