I want a simple formula (WITHOUT involving complex mathematics, i.e.,
partial differentiation, triple integration, or the like) for working
out the following. (If such a simple formula is not available, I can
accept, as an alternative, information based on actual experiments
made by reliable institutions, or published technical data from
manufacturers of ammunition):

Question:

For ammuniton (of bullet calibers ranging from 0.22 to 0.45 inclusive,
from popular manufacturers, with common gun powders being used
nowadays), how do I find out the volumetric expansion ratios (the
total volume of the hot gases created by a complete combustion of the
gun powder inside a bullet at the moment of detonation, compared to
the original volume of the gun powder inside the bullet), when the
bullets are fired in an open atmosphere under normal room temperature
of say 21 degrees Celsius? (I need this information for the above
range of bullets).