I always thought that a shotgun had a hollow sound. I could hear the other hunters shooting across the valley on opening day for pheasants.

https://en.wikipedia.org/wiki/Helmholtz_resonance

http://hyperphysics.phy-astr.gsu.edu/hbase/Waves/cavity.html#c4

0.7" diameter = 0.5^2" area.
Barrel length = 36"
Volume = 18 ^3"
speed of sound = 343 m/s

Using this calculator and these assumptions, the average 12 ga barrel would have a resonance of 60 Hz.

For those of you in Rio Linda, it sounds like the milk jug band.

There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. -Ernest Hemingway
The man who makes no mistakes does not usually make anything.-- Edward John Phelps