Why is a gun less accurate when the barrel heats up?

This sounds like two faces of the same coin - expansion due to heating. In the first set-up, the barrel gets hot because of the friction of the bullet and the exploding charge propelling it. The internal barrel diameter will increase slightly due to the excess heat, giving a chance for some of the force of the charge to leak past the bullet, less speed and momentum generally equals less accurate. With a relatively cold barrel and the heated sniper bullet mentioned, the reverse is true - a fractional increase in the bullet diameter will make a slightly tighter fit in the barrel - hence more accurate. Not sure how far to take this, as someone else may know, but if you seriously chilled the barrel, and added a hot bullet, chances might increase of the bullet jamming?