Handbook for Shooters and Reloaders

[John Lawson]

In that internal pressure from an ignited cartridge acts equally in all directions, bolt head pressure is equal to the presure exerted on the chamber walls. If there is any slack in bolt lockup, there will be some case rearward movement. At no time during the ignition of a cartridge does the bolt "thrust rearward" differently due solely to cartridge shape. Straightening out the walls of a case and making a similarly shaped chamber will DECREASE THE PRESSURE in the chamber at peak. This is only incidentally due to a change in wall shape. It is primarily due to an increase in water capacity of the case internally, resulting in less peak chamber pressure and therefore less thrust against the bolt face, chamber walls and bullet. And a very little change makes a very big difference with some powder types and loads; not so much with others.
Some early experimenters took advantage of the capacity increase to pack in more powder, resulting in what was called "overbore loading." We won't even go there.
Little was known about internal ballistics in the 1940's, and it has only very recently been explored in depth in publications such as Precision Shooting.
Very often, ballistic experimenters come up with the what of something but not the all essential why.

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