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With the Army Model 1903 and civilian models starting around number 225xxx, Colt reduced the barrel diameter and reamed the chambers to accept .38 Special. They didn't move the shoulder forward, they removed it entirely.
I have heard that they heat treated the cylinders at that time, but have not found anything in writing to that effect. The pressure increase is large in terms of percentage, but not so much in actual pressures - the .38 Long Colt runs 12k psi, the .38 Special 17k, and the cylinder would be perfectly adequate for the .38 Special even without heat treatment (as S&W proved).
FWIW, the S&W Model 1899 M&P cylinder is actually smaller in diameter than the Colt Army & Navy cylinder, 1.44" to 1.52".
I will note that the drilled through chambers will accept some factory .357 Magnum rounds, and that that cartridge has been fired in some of those guns without bursting the cylinder. Definitely NOT recommended, though!
Jim
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