Whereas many people discard used corrugated cardboard boxes, I treasure them like gold, as sheets of this stuff show the impacts of steel ammo very nicely indeed - not to mention the fact that the ammo rebounds between the suspended old bath towels I use and the piece of cardboard, and then falls to the ground to be easily collected later. Paper does not necessarily show the impacts, unless it is placed on a cardboard background (tried & tested).
Impacts on corrugated cardboard make a healthy noise too, which is an added fun factor. Maybe a microphone placed next to the target, and a good set of loudspeakers are something to try, but you might risk all your neighbors running for shelter, and calling the cops :hmm:. Just a crazy idea...let's move on.
Shooting tight groups on marker-drawn 1.5 inch circles at 10 yards requires that tough combination of proper stance and 150% concentration: yes, it's hard work in a sense, yet very rewarding when you go check out your group cluster between every string of shots, and see what you're hoping to see. Above all, it will give you clear feedback on what you may be doing wrong if your shots are off-center or erratic.
For anyone seeking to attend slingshot shooting tournaments, paper or cardboard is definitely the way to go in my opinion.