I posted on the alleged issue with 125 gr bullets in much more detail, but here's the cliff notes version:
1) The internet myth is that the shorter bullet lets gas around it that preheats the forcing cone and lets the bullet damage it. That fails basic thermodynamics given the very short time interval involved and the inability to transfer any significant heat in that time.
2) The issue is that:
a) lighter bullets have larger powder charges;
b) slow burning (for a handgun) colloidal ball powders are popular in the .357 Magnum;
c) Colloidal ball powders only partially burn in the case with the result that the abrasive un-burnt and partially burnt powder flows through the forcing cone along with hot plasma from what powder has burned.
d) This causes increased forcing cone erosion compared to a medium or medium fast burning (for a handgun) powder like Bullseye, Unique, BE-86, Universal Clays or one of the "Dot" powders.
e) Throat erosion creates small v shaped cuts in the forcing cone, and these cuts are stress risers. In the Model 19, the bottom of the forcing cone is milled away to create clearance for the crane. This results in a thin spot in the forcing cone at the 6 o'clock position and this combined with the stress riser created by forcing cone erosion will lead to formation of a crack in this area. In essence, the bottom of one of these v shaped cuts, just extends as a crack all the way to the outside of the barrel.
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Given that a 125 gr bullet uses around 21 grains of a colloidal ball powder like Win 296, compared to around 16 grains of the same powder for a 158 gr bullet, there's about 25% more powder flowing through the forcing cone. At best it represents a linear increase in forcing cone erosion. It's probably non linear as the increased charge and increased plasma probably creates a bit more than 25% of the forcing cone erosion.
In other words, the problem isn't the bullet, it's the larger powder charge, and more correctly the powder itself.
In if you never used anything other than a 158 gr bullet, those 16 grain charges over 25% more rounds will create just as much erosion.
The saving grace is that most people will never put enough .357 Magnum ammo through a Model 19, (or Model 13, Model 66, or one of the Model 10-6 revolvers in .357 Mag) to have an issue - but all the K frame .357 Magnum S&Ws are more at risk once forcing cone erosion is present.
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A related internet myth is that these slow powders that produce maximum velocity in a 6" or 8" barrel also produce more velocity in a 2 1/2 or 3" barrel. I've been chronographing loads for over 25 years and I've never observed that to be the case.
Those loads do however produce substantially more recoil, given that all the mass of the powder is expelled from the end of the barrel at about 3 times the velocity of the bullet. This means that a 20 gr load of 296 has 12 more grains of mass than an 8 grains of Unique, and will produce recoil equal to about 35 more grains of bullet weight when the velocities are the same.
This means that even though I can get about 1243 fps with a 125 gr bullet with 8.5 gr Unique in a 3" barrel, I get only 1132 fps with 19.5 grains of Win 296 (with both loads operating at about the same pressure and same percentage under maximum) but the Win 296 loads generates more recoil - 6.79 ft pounds compared to 5.05 ft pounds. That's 111 fps less velocity, but 34% more recoil with Win 296.
The problem of course is that if you have a shooter shoot both loads he'll swear up and down on a stack of bibles that the 296 load is faster, because it has much more recoil. If you show him the actual chronograph data he won't believe you.
So, slow burning powders are popular in .357 Magnum, even when it's actually costing velocity in a short barrel, and adding throat erosion for no useful purpose.
