Thank you guys for your kind words! I always amaized when I see some of your creations and what you can do with wood and metal. You guys inspired me to try it out. I have been here for a while and kept on reading some of your projects and learning about a lot of stuff. Still I have hard time with measurements in America:) -Inches and feet, sometimes confusing. But I am learning.
Jeremy, I made the pellet trap without any measurements but I just came home and I took the size down and it is: 25" wide, 12" tall and 14" deep. I have made the back wall under 45 degree angle- the reasons are obvious. I went to hardware store and I was really debating what thickness of steel to use and I went with 1/8 of an inch.And since it is under 45 degree angle it took my Condor with 32gr EunJins pretty well. I shot from 18 yards away and it made some dents as you can see but I made the trap for longer range, so it will hold up for a while. One more thing I did was to double the thickness of back wall with some extra piece of plywood in case of penetration. I also put steel plate on the bottom- I figured pellets will go down with force after hitting the back.
Kev, I do not know so much about the metals but if you talking about airgun in .22 cal then I think .125 steel thickness will work just fine especially if the back wall is under angle. IMO every back stop will eventually fail but .125 is enough, I do not know about firearm though.
Thank you guys