So then, the expansion ratio is one of the parameters used in selecting the powder with the proper burn rate, along with relative bullet weight. Yes?
Yes.
The expansion ratio is a factor in the relative difficulty of shoving the bullet forward � or, IOW, how (relatively) tightly or loosely it resists the expansion of the powder gas. The tighter that containment is, the faster the powder burns. A too-quick powder raises the pressure too fast � a too-slow powder doesn't burn fast enough to develop the desired pressure.
An extremely too-slow powder � a caseful of 50BMG in a 9mm Luger, say � may not even ignite as the primer pressure blows it out the muzzle.
An extremely too-fast powder � a caseful of Bullseye in a .340 Weatherby, for example � builds a straight, vertical pressure "curve" that doesn't reach its potential peak before the breech becomes a rapidly expanding cloud of itty-bitty pieces of jagged, high-velocity fragments.
Neither is good, but at least a faint
floop! is harmless.
Burning rate, BTW, is the technical term for how fast two opposite surfaces of a kernel of a designated powder burn toward each other
under a designated pressure, in units of distance per unit of time (microns per nanosecond, say). The technical term for what handloaders mean by "burning rate" is
quickness. The two concepts are related but neither synonymous nor identical.
Two
chemically identical powders with
different kernel dimensions would have the same burning rate but different quicknesses.
Two chemically different powders may have two very different burning rates but roughly identical quicknesses because of their different shapes and dimensions (sticks and spheres, for example).
