The temp gun is gonna give u different reading depending on what kind of surface you are pointing it at and how far away u are from the surface, different brands of temp gun vary wildly in accuracy. I would disregard the temp gun for now, you also need to remember that it takes a area and averages the temp for that zone, the farther away you are the bigger the zone gets, also uneven surfaces "can" disrupt its readings, this is because it uses reflected infrared light to measure the temp.

If you use a red heat bulb hanging over the tub I would use a rheostat to control the temp and put a digital thermometer probe under where the heat bulb is. I have used this setup before, I put a hide in the tub that was as tall as the tub itself so when i closed the lid it would fit completely snug in there, than the heat bulb would be hanging over the spot where the hide was, and my thermometer probe was inside the middle of the hide, the hide will absorb the heat from the lamp through the surface of the tub and the hide will then radiate the heat down to where the snake lies inside of the hide. This worked great for my bloods, you just need to realize that if the thermometer probe says 82 it is on the bottom and the heat will be warmer closer to the roof of the hide, I just set the rheostat to keep that temp at 82 where the probe was and this seemed to work great, and the cool side of the snakes tub would hang around 78-80, keep in mind the room where the tub is kept was a constant 76-77 degrees F.

PS I used a rubbermaid 12qt dish tub for a hide, I would cut down the rim so it would fit snug into the tub and I would cut a little entrance as well. I reccomend getting this in a black color as it seemed to warm up easier than the other colors, I can only guess this is because the black color absorbe the heat from the red heat bulb better than the lighter colors.

PPS sorry for the huge run on sentence, but I was trying to pack in as much info as I could.