have you tried shooting the 13 inch oak log with anything else , I found that even the lowly 30-30 with cast bullets in a not particularly hot round was going thru a 9 inch oak log
you need a tree wider than you are to call it cover when you start shooting rifles at it
softer woods , well it's going to be hard to find a tree big enough.
this isn't the movies where a door jam and a few 2x4's stop bullets
when you get done with all your testing , it would be interesting to see how many shots at the same spot it takes to get thru that 8 inch concrete. looking at the crater of a hole it left , i am thinking 3
I would bet that with a bandolier of enblocs you could make swiss cheese of that wall making it lousy cover
thank you and very nice job