Picher, you're not comparing two similar things in your ruler length to pressure point comments. But a pressure point on a barrel will indeed change the vibration characteristics although the fundamental frequency of the barrel and its harmonics remain constant although the amplitude will change.
The barrel's in a free-fixed holding condition when in a receiver and has a different fundamental frequency than a free-free one that's not fixed at one end. A pressure point on a free-free barrel will have a greater effect than on a fixed-free barrel.
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