I forget the absolute number off hand but lead deforms (Upsets) at a bit over 1400 psi times the Brinell Hardness number. The "Blow" of the burning powder hits the load from the back, therefore that's where the worst upsetting occurs. To upset the front after it exits the muzzle it has to have enough air resistance to exceed the deformation Threshold. A too Hard cast bullet fired from will lead the breech of the barrel in the cone because it Does Not upset & the gas cuts by it causing the leading. In the shotgun of course the wad is supposed to be sealing the gas from eroding the shot.
Maybe I've not studied all the "Books" some have but a lot years of fooling with a lot of things of a mechanical nature I have gained a bit more common Sense than a few here seem to have. Lay a short fat lead rod down on its side & another long a lick on one end slender one of the same weight. Hit them both a lick on one end with a hammer which covers their entire face & see which one deforms the most & where it occurs.
By the same token a shot load with a long shot column has more resistance per square inch to being moved than does a shorter column.
