Hiding Behind a Tree - Can It Stop Bullets?

In our test, we take some decent-sized pine tree trunks and shoot them with handguns, shotguns, and rifles. As expected, 9mm handgun and all shotgun rounds weren't able to penetrate the tree.
When we step it up to rifle rounds, both 5.56x45mm and 7.62x39mm were able to make it through without much hassle.
What surprised us all is that .308 failed to penetrate this tree despite having much more energy compared to 5.56 or 7.62 from an AK...
We think that because the .308 gets deformed when it enters the tree, it rotates, deviates from the path, and loses more energy.
What do you guys think? Why did the .308 struggle with this test?
Also, if you have any strange or interesting ideas for us to test, let us know in the comments below!