Pretty sure how rebuilder did it has come up before. that have industrial size ovens that can heat up the entire tube very evenly, to the point where the glass just lifts off. The issue with the heat gun is presumably un uneven heat causing stress for un equal expansion of the glass. I think the roundies are generally safter than the rectangles. Later rectangles had safety banding that would compress the perimeter. I the perimeter cant expand, then the tube can not explode along the perimeter, just inward from the implosion. After having one go off on me I tend to not mess with it anymore. just look for a decent CRT and live with it. Oh one more thing, if you are using heat clearly do not use a damp cloth to cool it down, again rapid uneven heat and cool is not what you want.
https://www.youtube.com/watch?v=lRJ0otqgmYs
check out the burnt shadow mask..