IIRC the Model 19 was designed using the old 158 grain .357 Magnum loads, a bit later the 125 grain and 110 grain loads became the fad and cracked forcing cones began to surface.
While not 100% sure of the technical reasons my caveman brain grasps it this way :
The heavier 158 grain bullet accelerated at a set speed where by the time it made the "jump" to the barrel the majority of gunpowder had already been burned,
I might be wrong but it would stand to reason that a lighter bullet would accelerate faster and make the "Jump" a bit sooner and those hot expanding gasses then cracked the Combat Magnums at its weakest spot in the 6 "O" clock position where the barrel is thinnest .
Im sure there is a bit more to it than that but this is just how my brain grasps it.
PS the .38 special at 800 fps is a very different round power and pressure wise than a .357 moving at 1300+ fps
It's a little more complicated than that with three separate factors coming into play.
1) The baseline conditions changed:
As someone noted above, when the .357 Magnum Model 19 was developed, the 158 gr LSWCHP was more or less the normal load and was launched via flake and extruded powders. At the same time, police departments almost universally practiced with .38 Special ammo and carried .357 magnum ammo. The end result was that the Model 19 was designed to be shot primarily with .38 Special, with only a small percentage of the total rounds fired over the life of the gun intended to be .357 Magnum.
However beginning in the late 1960s lawsuits started to be filed on the basis that officers were "undertrained" by the use of .38 Special in training, and a majority of police departments responded by using .357 Magnum for training. The end result was nearly 100% of the rounds fired by Model 19s were now full power .357 Magnum loads.
2) The 125 gr hollow point became popular as a law enforcement load.
Around the same time the training requirements change, so did the service load. The 125 gr load started to become the go to duty load for many law enforcement agencies, as it was very effective, and arguably had less recoil.
3) Around the same time Olin started producing ball powders for magnum pistol use.
Ball powders had been around since 1933 and started to see wide spread use in WWII in the .30 Carbine. In the early 1960s they became much more popular in part because they could be made from recycled WWII cannon powder being surplussed at the time. Ball powders were (and still are) inexpensive to make and a 10,000 pound production lot of ball powder can be produced in about 3 days compared to about 8 weeks for some extruded powders. The production lots can also be custom blended by a ballistician to deliver what ever burn rate characteristics the ammo maker wants.
In the late 1960's and early 1970s these powders started to become very common in .357 Magnum loads.
How Model 19s crack:
The Model 19 has a flat milled under the barrel to allow clearance for the crane and this creates a thin spot in the forcing cone, relative to the rest of the barrel. By itself, this doesn't cause any problems. However, once the forcing cone starts to erode, that erosion presents as small v shaped cuts in the inside of the forcing cone. Just like a crack in sheet metal, that sharp point at the bottom of the V creates a stress riser, and in the thinner section of the forcing cone at the 6 o'clock location, that v shaped cut and the resulting stress riser can cause a crack to form in the forcing cone all the way through to the outside of the barrel.
The "simple" explanation.
The 125 gr load took the blame primarily because it was the Model 19s used heavily by law enforcement agencies with near 100% round counts using .357 Magnum loads that started to encounter forcing cone cracks.
Since this was a new phenomenon, and the 125 gr load was the new load, people put 2 and 2 together and decided it was the 125 gr bullet causing the problem. Unfortunately it was an example of 2+2=5.
The "simple" explanation that has been around for decades to explain why more cracks were seen with 125 gr loads versus 158 gr loads was that the 125 gr bullet was shorter, and thus allowed gas to flow past the bullet and pre-heat the forcing cone before the shorter bullet arrived in the forcing cone and the "hot" forcing cone was then more subject to cracking.
That theory is "simple" but it won't survive contact with more than about 1 week of any credible thermodynamics course. To be brief, the millisecond or so between the arrival of the hot gas and the arrival of the bullet isn't enough time for the hot gas to transfer any significant heat to the forcing cone.
The not so simple explanation.
The root cause of the cracking in Model 19s is the erosion of the throat that then creates increased stress in the area of the forcing cone over the relief cut in the barrel.
The 125 gr bullet by itself does not cause more erosion. However in the big picture, the combination of the move to 125 gr loads, the move to the use of .357 Magnum for training and the switch to ball powders in magnum pistol loads all created the potential for much greater throat erosion.
Here's why:
1) Slow burning colloidal ball pistol powders have much heavier charges than flake pistol powders.
In many cases this is on the order of over a 100% increase. For example 9.5 grains of Unique was a max load for the .357 Mag with a 125 gr bullet in the early 1970s, compared to 17.2 grains of Win 630 or 21 grains of Win 296.
2) Contrary to popular belief, the slow burning colloidal ball pistol powders are not completely burned before the bullet exits the case.
If you've ever been pelted with bits of powder when shooting a .357 Magnum you're more than familiar with this phenomenon.
These much heavier powder charges also mean more mass flowing through the forcing cone - mass in the form of hot plasma mixed with partially burnt powder grains that are comparatively abrasive.
So now after the bullet has passed, you've got hot plasma flowing through the forcing cone carrying abrasive bits with it - twice as much of it, and a much higher percentage and mass of unburnt or partially burnt powder grains to provide the "grit" to go with the heat.
3) 125 gr bullets are lighter and use more powder than 158 gr bullets.
Using 296 as an example again, 21 grains would be used in a 125 gr load back in the day, but only 17 grains would be used in a 158 gr load in the same era. This represents a 20% reduction in mass and grit flowing through the forcing cone with a 158 gr bullet versus a 125 gr bullet.
In short:
The 125 gr "bullet" got the blame, because it was a new load and had come into more wide spread use shortly before cracked started to become an issue in the Model 19.
But it wasn't just the 125 gr bullet, and it had nothing to do with the length of the bullet. It was instead due to the increased erosion causes by the switch to much heavier charges of colloidal ball powders, which result in significantly more mass of hot plasma flowing through the forcing, cone, significantly more abrasion of the forcing cone, and in the case of the 125 gr bullet, even more mass flowing through the forcing cone with the 20% larger charges used relative to a 158 gr bullet.
Love your Model 19?
Stay with standard pressure .38 Special and .38 +P loads. Your Model 19 will last pretty much forever with standard pressure target loads and a steady diet of +P loads won't shorten it's life by any noticeable amount - particularly if you stay away from colloidal ball powders.
If you shoot .357 Magnum loads:
- Don't worry too much about the bullet weight (other than recognizing that maximum loads with lighter bullets use more powder and produce more erosion than heavier bullets).
- Avoid using slow burning colloidal ball powders like H110/Win 296, Win 630, etc.
- While it is technically a flattened spherical powder and not a colloidal ball powder, I'd also suggest staying away from 2400 as it's also a slow pistol powder that doesn't completely burn in the case, and will send a significant amount of gritty unburnt powder through the forcing cone.
- The same comment applies to Alliant's Power Pro 300-MP. It's a great .357 Magnum powder in a long barrel .357, but it's also a spherical ball powder and there's no free lunch.
- Choose a more moderate mid range load using a medium burning flake powder. It'll mean giving up some performance in a long barrel, but you won't lose anything in a short barrel and in some cases you'll get more velocity. Blue Dot is still a good choice and the faster Red Dot, Bullseye and Unique work as well as they ever did.
Be advised however that load data for some of them has been reduced over the years, so if you're using an old manual (which you often have to to find data on old powders) be sure to start with a moderate load and work up.
BE-86 looks interesting as it's in the Unique range for burn rate, but offers a lot more energy in a given charge weight.
Just keep in mind that shooting a Model 19 shouldn't be about squeezing maximum performance out of the .357 Magnum.
If you start having brass that sticks slightly in the chambers, you've probably gone too far and you'll want to back it down to the "non-stick" point.
