24 degrees would be fine IMO. Pretty sure mine is closer to 30 degrees with a 1/2" opening leading into the approximate 4" dia heavy wall tube. Care must be taken to make sure the tangent wall is slightly proud of the tube wall to avoid a protruding edge where bullet skips onto tube wall. I still use mine on occasion for primarily .22 pistol. Structurally it shows no signs of deterioration. I have also fired a few hundred .40 and 9mm into it with no issue. I NEVER shoot centerfire rifle at ANY steel targets. I always wear safety glasses and never shoot closer than about 20'-25'.
There is very dangerous risk of richochet if one hits a leading edge of this style of target.... This bullet trap has served me well over the years with tens of thousands of rounds shot into it, but I cannot be held responsible for anyone who chooses to replicate it.
Here is a quick sketch showing the basic idea behind my design. Use at your own risk.
Original post from 10 years ago.
http://forums.pelicanparts.com/off-topic-discussions/516406-just-finished-tested-my-prototype-indoor-bullet-trap.html
