[Steely Tom]

I think the wobble was caused by the spin of the bullet due to rifleing.

The bullet appeared to be almost perfectly bisected by the blade; not enough asymmetry present to excite such a strong vibration.

But the bullet was spinning pretty fast; it had to transfer all its angular momentum to the blade when it was cut. That’s why the blade edge vibrated in that “twisty” high-frequency mode.

[Moonman62]

That makes a lot of sense.