Full 3-D Finite Element Modal Analysis of a 270 Win Barrel That
Had a Double Peak Pressure Trace

This is the pressure vs. time plot for the 270 Win barrel.
The second pressure peak occurs with a width of about 0.3 ms.
If is were part of a wave form with a 1/2 wave of 0.3 ms, then
the frequency of the wave would be approximately 1667 Hz.
The mode closest to that frequency is the mode 5 in torsion. See Below.

Info from Joe:

The barrel is a Lothar Walther SS. I believe it is a # 1300 contour, free floated from the front of the receiver ring (M70 action) to the muzzle. The chamber is unsupported by bedding, I know one day I'll bed it right.
length = 25.75"
diameter @ receiver = 1.202"
@ 4" from receiver =.920
@ 8" =.812"
@ 12" =.770
@ 16" =.727
@ 20" = .683"
@ 24" =.651"
Muzzle diameter = .650"

Using Joe's information, I made a 3-D mesh of the barrel and performed a modal analysis of the first few mode shapes and frequencies. The breach end of the barrel is rigidly fixed. The material is 416 Stainless Steel. The deformations in the mode shapes displayed are arbitrary large so one can clearly view the shapes. The real hardware does not deform this amount.

Table 1: Mode Frequency and Shape Summary

This is the Mode 1 frequency of approximately 52.5 Hz. It is the simple bending mode.

The Mode 2 has a node near the muzzle and the frequency is about 250.5 Hz.

Mode 3 has 2 nodes and a frequency of about 633 Hz.

Mode 4 has 3 nodes with a frequency of about 1185 Hz.

Mode 5 is a torsional mode and has a frequency of about 1683 Hz.

Here is another view of Mode 5 with the mesh superimposed on the model so one can view the angular displacements.
Possible, the bullet traveling down the barrel could be winding up this mode like a clock spring and then releasing it when the bullet exits.

Mode 7 is the axial extension/compression at about 2329 Hz.

I don't see anything in the Modal Analysis that jumps out at one as being the cause or leading to the second pressure pulse.

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