It seems like there might be a fairly serious bug in both the M1A1 & M1A2. If you fire at the ring where the turret meets the hull, there appears to be a small area where the rounds pass through the entire model without impacting & causing damage. It seems to occur consistently between both camouflage variants of both tanks. I've not noticed it in any other vehicle thus far but I've only done limited extra testing so far.
Video of the bug in action