Many readers may know that, often, in such loads, use of a smaller-diameter expander ball results in greater peak pressure and velocity (within reason, use of a smaller expander increases resulting bullet-to-neck tension and hence increases resistance to bullet movement). Similarly, experienced handloaders have long recognized that in 38 Special midrange loadings, slight differences in crimp alter internal ballistics: less crimp, less pressure; more crimp, more pressure.
Excepting one fact, one could rightly argue that greater crimp or greater neck tension delays bullet movement longer, after primer explosion, so that chamber pressure is higher before bullet begins to move, equally as well as one could argue that greater resistance to bullet movement reduces primer-induced bullet movement, so that boiler room is smaller when charge ignites. That one restricting fact: Otherwise identical loads always show significant primer-induced bullet movement when tested without any charge! – the hotter the primer (i.e., CCI-350), the greater the bullet movement. Evidently, in these particular loads, the main difference is how far the primer moves the bullet before propellant ignition and subsequent combustion generates significant pressure.
These examples are sufficient to persuade this author that smokeless propellant ignition is significantly delayed, with respect to primer blast. These tests would seem sufficient to demonstrate that, in many loads, force from the primer blast does move the bullet before propellant ignition generates significant chamber pressure. For the past three decades, this author has pondered how to go about proving (or disproving!) this contention.