When a cast bullet extends into the case, it has the base, and shank of the bullet in contact with the powder. When that powder burns it becomes a gas. This gas is going to work on that exposed surface, and its going to deform it. When a bullet obdurates, it shortens and widens to fill an available area, in this case, the shoulder of the cartridge brass. There is no guarantee that this process will happen uniformly. What do you think happens to accuracy when a deformed bullet leaves the bore?
No, it is not the same as the gases operating on the base of a bullet, supported by the neck of the case. Here, the force of the expanding gas acts purely to launch the bullet. In the example of using a long bullet, and any 190 grain cast bullet is going to have a longer shank than neck length, given correct OAL in the 30WCF, those same gases are allowed to operate on the entire exposed area of the cast bullet. This will, in fact, negatively affect accuracy, can easily lead to leading, and pressure issues, as stated above.
