I seent this on Accurateshooter.com
https://apps.dtic.mil/sti/pdfs/ADA622138.pdf
. The ullage provides a certain amount of freedom
for the propellant bed to compress and move during the initial stages of the interior
ballistic event. With minimal ullage available, the propellant merely transmits the
primer force to the projectile, acting much like an incompressible fluid. Conversely,
a cartridge with more ullage will create a situation where the initial motion of
projectile and propellant act more in unison.
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