I was out target shooting with a friend of mine, and about 2 hours into it I commented how my accuracy seems to be going down the longer we shoot. He (a fire arms instructor in the Navy, so I tend to believe him) said when a barrel heats up, the gun becomes less accurate.
On a side note, he said that when snipers are going for long (~1 mile) long shots. they take out the magazine and leave it in the sun allowing the ammo to heat. They will then load a single bullet at a time because, although a heated barrel will cause a decrease in accuracy, a heated bullet will be more accurate.
Assuming this is true, why would that happen? Is the heated barrel expanding and causing some of the gas to escape around the bullet? And would a heated bullet expand to cause more pressure (more speed/spin?)