I use newspapers rolled tight into a ball and wrapped in duct tape, if you press it really hard, it is not that bad..and very easy and fast to make

Use clean white paper as outer layer.

Willeke

Put it in a (clean white) sock?

Willeke

My favorite indoor ammo is: Take 2 sheets of printer paper...then crumple them together into a big ball (try to get it as small as possible)... and then wrap the whole ball with 2 or 3 layers of duct tape...Done...it has enough weight to be consistent, and it doesn't really ricochet...
Derek

I don't think I keep up with the topic and is it indoor ammo anymore, but letme tell :

This one time, instead of making homework I entered my workshop/workstall etc. and made a new sling. I was eager to try it, but it was dark, cold and unrocky outside. I figured out, that I needed something silent (mother sleeping). So I took a used up 9V Battery (the squarey one), inserted it in leather and put it in a cut piece of the thick spongy sponge You cover pipes with (an isolating one). I spinned, whammed at a ceramic fireplace but hey - as much sound as a tennis ball bouncing.

It Ricochets so it's not good, but the point is it was well-weighted and really silent.

I use a 1 3/8" diameter neoprene rubber ball purchased from McMaster-Carr industrial supply.  In the past they have cost about a dollar apiece and have good mass, flight characteristics and bounce off most walls.  If you want to be aggressive with them it is best to find an interior brick wall however.  They are also white which makes them good to find if you venture outside with them.

GN