look its easy. look at wind tunnles. how do they get air velocities in excess of 14X the speed of sound.

simple let air expand at its fastes rate (around the speed of sound), and then channel this through a cone shaped nozzel. using the correct ratio for the change in dimater, and a high enough starting pressure you can obtain very high speeds.

Its true that the the air temp drops greatly which may affect the spped of sound localy. but as soon as the pellet exits the barrel it is travely through standard temp and pressure. so they can all be ignored.

speeds in excess of 3X speed pf sound can be produced in open ended wind tunnles. so a supersonice air gun could be produced.

Has one been made is another matter, im a sub 12ftlb guy