Air rifles like flame thrower use a very high pressure to launch a projectile down the barrel.
All that pressure escaping at once can create a hefty acoustic pressure, what you hear as a 'bang' or 'pop'.
No projectile, that burst of pressure could be faster than sound creating a shock wave that can be loud.
Usually this is.below that speed when a projectile has to be pushed down a barrel so not as loud.
An early .22 full length barrel I had, just would take the pressure of shooting the short and hardly more than a mild 'pop'. Load with the long rifle, it was quite loud.
My 22-250 with 14 inch barrel, handles over 60,000 PSI chamber pressure, all that escaping at once is surely the loudest gun in the county.