If you aren't sure what people are meaning by "bullet setback", it is when a round gets chambered and the bullet is slightly pushed back into the case. After repeated loading/unloading of the same bullet, the movement can be substantial. When the setback becomes significant, the reduced case volume increases the pressure when the round fires. If the pressure is too high, it can cause catastrophic failure of the gun, otherwise known as a "KA-BOOM".
Here are some pictures from a message thread discussing
this topic at Ohioans For Concealed Carry.
The 4th from the left is set back and so is the one in the middle with the black line on it.
Here is an example of bullet setback with 35 thousandths difference of COAL.
When we hear of gun KA-BOOMS at the range, most of the time we really don't know what caused it. I'm convinced that bullet setback is the culprit more often than people think.