Quote:
Originally Posted by Rule3
/...I also did not test Blue Dot due to Alliants warning, and did not use a SW K frame due to the "internet chatter" of forcing cone erosion with light bullets.
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That's another interesting subject, and another area where myth seems to be common, in an in contradiction to physics.
The popular "wisdom" is that 125 gr .357 Magnum loads produce forcing cone erosion in K frame revolvers - and more specifically the Model 19.
Part of that is historical coincidence. The Model 19 was developed to be the optimum police revolver, but was designed at a time when police departments used .357 Magnum for duty ammunition but practiced with .38 Special. As a result the Model 19 was designed to tolerate full power .357 Magnum loads for duty use, but was designed with .38 Special practice in mind with a ratio of .38 Special practice ammunition to .357 Magnum duty ammunition of perhaps 100 to 1.
However in the 1960s police departments started to get sued for under training their police officers by using .38 Special for practice, with the result that most departments started using .357 Magnum ammunition for practice. This meant that these Model 19s (and by then the Model 66) were now seeing a steady diet of full power .357 magnum ammunition.
Around the same time period the 125 gr JHP became very popular for police use, with the result that it was also the 125 gr JHP load that started to be used exclusively in the Model 19 and Model 66 revolvers in police use.
When some forcing cones started to crack, the 125 gr load got the blame, and the popular explanation, then and now, was that the shorter bullet allowed gas to get past the bullet and preheat the forcing cone before the bullet arrived, resulting in the bullet damaging the forcing cone. The problem with that explanation, then and now, is simply that the thermodynamics doesn't work. There just isn't enough time for any gasses ahead of the shorter bullet to transfer any significant heat to the forcing cone - certainly not enough to superheat it to the point it would be damaged by the bullet impacting it.
However, the "it's the fault of the 125 gr ammo theory was partially right, just for entirely different reasons. Ammunition companies used (and still use) colloidal ball powders (more or less equivalent to the canister grade reloading powders like H110/Win 296) in their .357 Magnum loads, Part of it is economics as these powders are quick, easy and inexpensive to make and since it's a wet process it's also comparatively safe. Part of it is the fact that these powders do produce high velocities in long (6" to 8") barrels. Part of it is just that these heavy weight powders charges also produce satisfying heavy recoil that convince customers that they have a truly magnum load.
The problem with these large charges of slow burning colloidal powders in a revolver is that that it results in a large mass of partially burnt and rather abrasive powder flowing through the forcing cone, along with the superheated plasma from the powder that has burned. That hot, abrasive, partially burnt powder is in contact with the forcing cone for much longer period of time (albeit still in milliseconds) than any gas ahead of the bullet. As a consequence the .357 Magnum loads using these powders did have a tendency toward excessive throat erosion (and they still do).
However, the problem is worse with 125 gr bullets, just due to the greater size of the powder charge. If you look at maximum loads in data books from the time period when 296 and H110 first appeared on the market, you'll find maximum loads around 21 gr for the 125 gr bullet and around 17 gr for the 158 gr bullet. 21 grains is a 20% increase over a 17 grain load, which simply put means 4 shots with a 125 gr bullet will have as much powder flowing through the forcing cone as 5 shots with a 158 grain bullet, producing the same amount of forcing cone erosion in 4 shots rather than in 5.
Early 296 data also showed 24 gr max loads for the 110 gr bullet, which was a 30% increase over the powder charge with a 158 gr bullet, with a commensurate increase in forcing cone erosion. But, since the bullet was shorter, it was easy for shooters to blame the shorter bullet rather than the increased charge of colloidal ball powder.
Forcing cone erosion matters as it creates V shaped cuts in the forcing cone. The sharp end of the V creates a stress riser. That stress riser then causes cracks in the K frame .357 revolvers at the bottom of the forcing cone where the barrel has been milled flat to allow clearance for the crane. There's less metal in that area, so the increased stress from a V shaped cut in that area is much more likely to cause a crack to form.
Forcing cone erosion is not unique to the K frame either. Ruger developed their Service Six, Security Six and Speed Six revolvers in part to address the cracking issues experienced by a small percentage of S&W K frames in police service, and as such the Six series revolvers had a thicker forcing cone and a larger frame that eliminated any need for a cut - more or less the same approach S&W took with the L- frame revolvers. However, I've seen high round count Six series and L frame .357 Mag revolvers that have significant forcing cone erosion - and I've passed on them.
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In short, if you are going to shoot your .357 magnum a lot, you'll have a lot less forcing cone erosion, and a lot less potential for a crack, if you stay away from colloidal ball powders like H110/296, and if you do use them, reserve their use for heavier 140 and 158 grain bullet weights where they actually offer some advantage.
The lighter 110-125 gr loads are where the performance advantages of Unique becomes relevant as it gives up nothing with a 125 gr bullet in a 4" to 6" barrel and almost always has an advantage in a 2" to 3" barrel.
With charge weights that are half that of Win 296 and with Unique being a faster burning and much less abrasive powder, forcing cone erosion is greatly reduced. It's a win-win situation.