I doubt that an extra 100 fps that is generated by most factory +P ammo is going to damage the gun. If the frame cracks like the AirWeight J-Frames used to it probably would have cracked with standard loads.

I do agree with the other poster that measurments should be taken of the cylinder gap and headspace to se how much endshake is generated buy the pounding. It is to bad you don't have a second M12 that is in equal condition to shoot them side by side, one with +P and one with standard loads to see what the difference is at the end of the test.

There would probably also be a difference depending on the ammo used. Winchester +P .38 Special is not even close to CorBon +P ammo in pressure or velocity. Same with Buffalo bore. Their 158 cast SWC is going 1000 fps from a 2" gun vs. a Federal, Winchester or Remington +P 158 that is only doing 790 fps...

Let us know how things are progressing.

Bob