I'm thinking this guy's a few reloads short of a 'hold muh beer' alert.
Read the part about how the touchhole or fuse passage leads to a small "chamber" in the center that holds the powder in a single spot, rather than letting it cover the whole 8.5" bore; he's using the same high-low pressure chamber technique that lets aluminum-barrelled weapons like the M79 and tear gas grenade launchers function with aluminum or plastic barrels. In their case the pressure is contained in a high-strrength section of the cartridge case head; in the case of these guns, in a pressure chamber in the base of the barrel.
With the 40mm M79/M203 round, that works out to around 3000 PSI contained within the aluminum high-pressure containment chamber in the base of the shell, which ruptures a blowout disc and is then vented against the bottom of the 40mm full-bore diameter projectile. Once the projectile overcomes its inertia and begins moving, the pressure is not only reduced due to the correspondingly lareger area of the projectile's base, but as it moves further up the bore, the initial pressure drops off to even less. Same idea with the bowling ball launchers. Or with some of them, anyway.
-archy-/-