"Proper paper" in the context of slinging.org likely refers to:
I cannot play the video to confirm, but I suggest checking the slinging.org forum for the thread matching that attachment name for the exact context, as attachments there often depict experiments with different materials. VID-20220418-WA0011mp4
Documentation or instructions referenced within the forum attachments, potentially describing the use of a "proper" projectile. "Proper paper" in the context of slinging