[mrerick]

It's basic internal ballistics. Look up "freebore' and "throat". This is how the initial pressure peak is managed in a modern handgun or rifle chamber. For a reference, see George Nonte's classic book on "Pistolsmithing".

Consider this analogy... Driving a nail into wood is similar to pushing a bullet up the leade of a barrel into the rifling. When you drive a nail, do you put the mass of the hammer against the nail head and push, or do you swing the hammer's mass across a distance of air and strike the nail into the wood? You can still drive the nail without using the momentum and mass of the hammer, but it takes considerably more pressure building against the hammer before the nail will start to overcome the friction and start to move.

Without adequate freebore, the bullet won't have the momentum to easily engage the leade and start down the rifling. Pressure will continue building in the chamber until the movement finally provides more volume to expand in.

This can be measured in an instrumented chamber.

If a cartridge OAL is too long, the initial pressure peak will be higher and sometimes dangerous.

BTW, Luger was experimenting with the chamber of his design from the beginning. There is a step in his 9mm Chamber as well as an oblation ring to help with the seal and release of the case.