To add to Roy's excellent post, note that these wave pressures do not give an increase in pressure over the total bbl area behind the charge, but a localized one when the moving wave pressure is checked by the charge. This is very similar to a ring bulge or burst from an obstruction. When the moving charge is checked the gas in effect piles up at the base of the wad & creates a local pressure spike, thus the "ring" bulge, or if severe enough a burst. As noted above if the obstruction is not severe enough to cause a burst, only a bulge, wave pressures may be set up resulting in more than one ring.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra