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mes228, the most common reason point of impact moves as barrels heat up is how they're fit to the receiver. Unless the receiver's faced square with the barrel tenon thread axis, the barrel will have one spot on its shoulder where it's hardest against the receiver. As the barrel and action heats up, that point puts a stress line in the barrel at that place causing it to whip differently.
In second place among reasons, the barrel's not stress relieved properly so it bends as it heats up.
Down in third place is on bolt action rifles with epoxy bedding under the chamber area. As the barrel heats up, there's more pressure on the barrel at that point as it expands. But it's usually not until several round are fired and the barrel heats up quite a bit.
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