Not so much the gun as the powder. Cold powder has been demonstrated to produce lower velocities in other tests and my own testing is consistent with that.
The sample size is too small to be sure, but it even looks as though the warmth generated by each shot caused the subsequent shot to be a little faster.
This load has been well proven in laboratory conditions. It consistently expands well in bare gelatin and through heavy clothing. The problem is, I believe all professional testing is done at around 70 degrees.
As far as water goes, it tends to produce the greatest upset that is possible with a bullet. Gelatin and tissue generally show somewhat less expansion than water does. Water beats the crap out of nothing but if you're going to use water, it's best to use a line of milk jugs or plastic bags (Fackler box) and denim and to bear in mind that it's not quite as accurate as properly calibrated gelatin.