I'm going to be shooting on an open flat field. Although there is nothing past the target area for over a mile, I know the importance of a backstop anyways for safety and legal reasons.
Normally I've been able to use large dirt or gravel mounds for backstops, but I won't have this option here. Additionally, I don't have any plate or sheet steel available.
I DO have lots of cedar logs on site, 10" diameter, 8 feet long. I can build a retaining-type wall with them to serve as a backstop.
How thick should it reasonably be in order to be "more than sufficiently safe"?
I'm shooting .22lr at 400 yards. I know at point blank range a .22 will penetrate completely through one log. I know the .22's velocity at 400 yards is much less than point blank, so...is a backstop two logs thick sufficient?
What say you?