Howdy
I got that graph from a ballistics technician years ago. I have posted it on several gun boards over the years.
I have posted it again here.
The graph represents the pressure curves of two charges of powder used in a 12 gauge shotgun to accelerate 1 1/8 ounces of shot to 1200 fps. Notice it specifies this is a 3 Dram Equivalent load, which is what 82 grains of Black Powder would be.
The problem with this chart is it does not specify exactly what Smokeless powder was used.
But we can still generalize a few things from the chart. Being a shotgun load, the Smokeless powder is going to be a fairly fast powder.
The important thing to notice in this chart is how much less pressure was developed with the Black Powder charge AND how much more spread out the curve is in duration.
Notice the Smokeless curve is a sharp spike, peaking and declining much more quickly than the Black Powder charge. That is the key. The older steel (and iron) in Black Powder era guns was not able to take the sharp spike in pressure as well as modern steel can. Even if the over all Smokeless pressure had been less, the spike would still have been very narrow. This tends to impart a 'shock' to the metal, that older steel might now be able to absorb without shattering.
Think of attaching a rope to your car to pull another car. If you accelerate very slowly and gradually, chances are the rope will not break and you will get the other car moving. On the other hand, if you start with a jerk, the rope may not be able to withstand the sudden increase in force, and may snap. Not a perfect analogy, but you get the idea.
The older steel or iron of a Black Powder era gun may not be able to take the sudden shock of the sharp pressure spike.
I argue about this with a guy on the S&W forum all the time, and it is indeed possible to formulate Smokeless charges with certain slow burning powders that will have a gentler curve and probably will not hurt the older steel. But those powders are not ones that are generally used with revolver cartridges. This guy has submitted formulas that he thinks are safe in the old guns, but I do not have pressure equipment to test them, and frankly, I don't want to risk my antiques on his say so, so I only shoot my old, 19th Century revolvers with Black Powder.
Rifles are a different story. The cross section of the chamber wall of a rifle is much thicker than the thinnest cross section of a revolver cylinder. As I have said many times, it is the cylinder of a revolver that must withstand the pressure generated when a cartridge fires. Not the barrel, and not the frame. When a revolver blows up, it is almost always the cylinder that bursts, not the barrel. The cross section of the chamber wall of a rifle is thicker than the weakest point on a revolver cylinder. I have several 19th Century Winchesters, and although I only shoot them with Black Powder now, I would not hesitate to shoot most of them with mild Smokeless loads if I wanted to. The 1892 Winchesters anyway. They have a very strong action. I would not want to be firing Smokeless ammo in my 1873 rifles with their weaker toggle link lock up.
Colt did not factory warranty the Single Action Army for Smokeless powder until 1900. Any Colt made before 1900 should not be fired with Smokeless powder. However, that is not to say that the first time an old gun is fired with Smokeless powder it is going to blow up. Obviously many old guns were fired with Smokeless powder over the years, and they have not blown up. Still, I stick to the rule and will not fire Smokeless in any of my old guns.
Around 1901 Colt began marking the trigger guards their revolvers with a VP in an inverted triangle. VP stands for Verified Proof and its Colt's way of saying the gun has been proofed for Smokeless powder. This Bisley Colt was made in 1908.
Smith and Wesson is a bit harder to pin down. Several of their very early 20th Century catalogs warn against using Smokeless powder. They hedge their bets a little bit by claiming they cannot guarantee the reloader will exercise the proper care in reloading with the 'new' Smokeless powders. After a while S&W relented, but I can't give you an exact date.
Iver Johnson completely redesigned their line of revolvers at the turn of the Century, using better steel, for Smokeless powder. They also completely redesigned the mechanism, and it is easy to tell a Black Powder era IJ from a Smokeless one.
This is a Smokeless powder Iver Johnson. Notice the little owl on the grips faces backwards. Notice the shape of the cylinder locking slots, how they have a straight edge on both sides, meaning the bolt completely locks the cylinder in battery. Lastly, if the grips are removed, the hammer spring will be a coil spring.
This is a Black Powder era Iver Johnson. Notice the little owl faces forward. Notice the shape of the cylinder locking slots. Only one straight edge, the hand prevents the cylinder from rolling backwards. And if the grips are removed, the hammer spring is a leaf spring.
You will often hear that modern ammunition companies purposely load some of the old cartridges, such as 32 S&W or 38 S&W to low pressures because there are so many old guns out there. I am skeptical of this because as I stated earlier it is not just the amplitude of the pressure curve that matters, it is the the length of time that matters too. I have lots of old revolvers that chamber 32 S&W and 38 S&W. I do not trust modern ammunition companies to have a gentle enough pressure curve to fire in my antiques. My rule of thumb is the same as the Colt rule of thumb. I doubt if S&W had access to any better steels than Colt did at the end of the 19th Century. My 19th Century Smiths only get shot with Black Powder.
Others can do as they choose.
I believe somebody mentioned Trail Boss. Trail Boss is absolutely not a Black Powder substitute. It is a modern Smokeless powder with a very fast pressure curve. It is unsuited for use in 19th Century revolvers. Trail Boss was developed for Cowboy Action Shooting because many shooters load the old, large capacity cartridges such as 45 Colt way down to recoil like a light 38 Special. Put a small charge of Unique or any other common pistol powder in a big case like the 45 Colt or 44-40 and you run the risk of poor ignition and spotty performance because there is so much empty air space in the cartridge. That is why Trailboss was developed, the big, fluffy donut shaped grains take up a lot of space in the big old cartridges, leaving less empty airspace, resulting in more a consistent powder burn. But Trailboss is much too fast a powder to be considered a Black Powder Substitute.
While I am on the soapbox, somebody mentioned the old thing about how slowly a trail of Black Powder burns. Not a good analogy to what it does in a cartridge. When you pour out a trail of Black Powder, like they did in the old Western movies, the flame has to jump across the airspace separating each of the powder grains. Not at all analogous to how it burns packed into a cartridge.
P.S. If you look at the front of each of those pressure curves, you will see a little bump. More of a little spike with the Smokeless curve. Those are the little pressure spikes created as the primers ignite.