Check out this awesome video to find out just how bulletproof wood is.
In this video a .308 and a .300 Winchester magnum are fired from an Armalite AR-10 and have to punch through several different slabs of wood ranging from 1 inches thick, up to 4×4’s.
The first few times he fires, the bullets go through all of the slabs of wood and comes out the other side. The bullet definitely slows down while passing through them, but still manages to get past more then 10 boards.
After a few more shots and thicker slabs of wood in place, he realizes that even though more boards are being placed down, they bullets are still going through. When the slabs are not spaced out and instead put back to back, they are much more effective and only a few of them are penetrated.
But nonetheless, wood is not very bulletproof.