It was not an issue of the rifle heating up or getting fouled. The low impact was actually the second shot (of this string) and landed on a black area of the target which I couldn't see from the bench. I fired two more to get a 3-shot group before retrieving the target. That's when I noticed the low shot.
One thing I forgot to mention - I chronographed the 250gr TMZ's & a 82.5gr weighed charge of BlackHorn 209, which is about 120gr V measurement. Using the original Barnes sabot the bullets registered 2,015 ft/sec. Using the MMP 3P-EZ the velocity dropped by 40 ft/sec.
I need to try the TMZ's again with the T/C Super Glide sabots to get a better idea of the potential accuracy with this combo. If I could RELIABLY shoot a 4.5" group @ 200yds this would still make a decent whitetail load, not exciting but sufficient.
All in all, the 250gr Shockwave in its original sabot shoots better.
As long as the three in the 4.5" group were fired consecutively, I wouldn't worry about the 4th. How many times are you going to shoot and reload 4 times in a row at an animal anyway? 1 shot is good, 2 shots is ok, 3 shots mean trouble, 4 shots is a lost cause.