Gopher;
I had almost the same type set-up as you at my prior house (current house allows me to shoot out to 300 yards into a hillside backstop
). The only thing different was I had two rows of railroad crossties. I tested the penetration with a .375H&H Mag loaded with hot Barnes-X bullets fired from 7 yards. Those did not come out the back.
A word though. As you shoot the ties will be whittled away. Eventually it will get to the point where penetration will happen more and more. (this was the reason for my 2 rows). The ties will eventually have to be replaced. Just prior to my move my some of my front ties could have used replaceing but hell... I was moving so why. The backstop's second row was still intact and stopping bullets but if the front ones were not replaced eventually the back ones would cease to stop bullets.
------------------
Schmit, GySgt, USMC(Ret)
NRA Life, Lodge 1201-UOSSS
"Si vis Pacem Para Bellum"