@Bart B... Not disagreeing but to clarify... Given that nodes antinodes remain the same for one mass vs another, ei: frequency, Amplitude is to the largest extent the only variable and is affected by the force exerted upon it from the inside or the outside of the barrel. However frequency is a product of how much stress is resident or "built in" (like stretching a guitar string)... so bedding, up-pressure contributes to installing that. Therefore One influences the other. I'm a guitar player (of some 40 years) and it is possible to pluck/strike a string with a given tension (built in) and extract different frequencies depending on how hard you hit it. It is possible essentially to stiffen a barrel by shooting lighter bullets through it.