[SJSAMPLE]

Check out the MythBusters episode on water stopping bullets.
Every round, including the .50cal, fragmented immediately upon penetrating the water. That's the main reason none of the rounds penetrated 6' under water. If those same rounds had been going slower, they might not have fragmented and might have penetrated 6' of water intact.

I was specifically speaking about 5.56mm (M193 and M855), which I've never seen fail to fragment in water or ballistic gelatin. Combat surgeons are retrieving fragments of 5.56mm, never whole.

[King Prout]

I saw that episode. I recall something different: rifle rounds penetrated the target all the way at the bottom of the water column, and a shotgun slug shattered the tank through transmitted shock.

[SFC Chromey]

Check out the MythBusters episode on water stopping bullets.

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I have a theory about that. I think if they were shooting at a more perpendicular angle there may have been a different outcome.