I have not tried it.

I would be reluctant to do so.

Toilet paper is cellulose; highly processed to separate the fibers which are then laid up again as a non-woven fabric. As a result, it readily combusts with a modest ash fraction and anything that doesn't combust is readily ejected from the barrel.

Polystyrene trays are a closed cell form form of the plastic. To completely combust, it must have high temperatures in the presence of air. As large quantities of air are not present in a gun barrel, the combustion will be incompete leaving behind an ash fraction consisting of various hydrocarbons.

Since toilet paper is working well for you, is readily available, is inexpensive and the alternatives offer no significant advantage, why change?