... Personally, I have seen approximately 250 fps difference between the first shot from a cold bore and the third shot as the barrel heats in a 26 inch Model 70 Classic sporter in 264 using 140 gr partitions in combination with RL 25 (early production). The fast loads were accompanied by blown primers and case heads expanded so much they would not fit into the shellholder on the reloading press.
I recorded, the same data on two different days with this load. Shot #1 safe and sane at near 3100 fps, second hitting 3200 fps, and third exceeding 3300 fps.
Never heard or seen anything like that. Even shooting high intensity cartridges in pencil-thin barrels in warm weather. But I guess one never stops learning.
