"I am no engineer, but it seems improbable to me that a manufacturer as prestigous as S&W would design a weapon where the margins of safety and service life would be significantly affected by the difference in standard and +P pressures - within reason, of course. But, as I said, I am no engineer and certainly have nothing to base my opinion on - other than my own, very limited practices. It will be interesting to see what your test proves."

The S&W model 12 was NOT designed for +P for the simple reason that the +P specification did not exist at the time of manufacture.
On the other hand, I AM an Engineer with Graduate study in Metalurgy.
BTW, be aware that your 'test' will only be a one time data point and the results should not be used to predict the future for similar firearms. Frankly to subject ANY product where failure can lead to injury, to tests that exceed the design specifications without the manufactures certified approval is foolhardy. -Dick