if you seat the bullets out to touch the lands the back of the case head will be pushing on the bolt face. slow burning powders will not create enough pressure if the powder is slow burning enough.
when the firing pin contacts the primer it pushes the cartridge forward till the shoulder contacts the chamber. firing pin force is enough to slide the case neck over the bullet until the shoulder contacts the chamber and detonation occurs. by using a compressed charge with slow bulky powder the case cannot slide over the bullet as the compressed powder charge won't allow it.the case head will remain on the bolt face and the expansion will be in the shoulder area blowing forward as it now has room to move.