JEFFJ13 you can do it just fine with the 1/4" plate. Angle it 45deg to the ground, place 1/2" plywood sides to catch bullet splatter. Have sand or pea gravel on the ground to trap the bullet. Have at least a 2x4 as the front lip. Get a high velocity fan or blower at your back and have an in/out ventilation system that will carry the lead dust away from the shooter and out the building.
Here in Alaska many people have 25ft shooting basements. Mine is a 3x3ft 3/8" mild steel plate, and can stop any jacketed non-magnum round no sweat. Hard cast magnum rounds are OK too.
A friend uses a different system which is a two foot thick hopper of pea gravel, no steel at all. Pistol bullets cannot penetrate 1 foot of pea gravel. The bullets impact the gravel and the energy converts it to dust. The pea gravel level continually falls, so it must be constantly refilled. The backing is plywood and the front is plywood with a double thick cardboard cutout over the target area. With a fresh piece of cardboard in place, very little of the gravel comes out. Its very safe, and deadens the noise.
