Miles, straight-line penetration in expanding bullets is primarily due to the length of the remaining shank after expansion, not what the bullet is made of. This has been proven many times in testing.
Monolithic bullets do tend to have longer remaining shanks, but so do many lead-cored expanding bullets, such as North Forks and Trophy Bondeds, because they have solid shanks with a small amount of lead in the nose. The heavier Nosler Partitions that have the partition further forward also tend penetrate very straight, because they end up with longer shanks, and so did the Fail Safe bullets, which had a lead core in the rear end protected by a steel cap. I've seen all those bullets penetrate several feet in a straight line in large animals like elk, moose and Cape buffalo. But occasionally have seen even long-shanked bullets deflect, whether monolithic or lead-cored, after hitting very heavy bone.
