It kind of depends on the mount.
If the targets are free to swing backwards when hit, (preferably deflecting rounds toward the ground), they generally don't have to be as thick.
Mine are 3/8" - 8" dia - and hold up quite well to .45ACP and normal pistol .44M.
They hang below an axle and spin when hit solidly.
If your backstop was free at the bottom to swing backwards, that would definitely help.
As long as it didn't twist sideways and cause ricochets.
Lock the doors, they're coming in the windows.